Add database model for defining custom units (#5268)

* Add database model for defining custom units

- Database model
- DRF serializer
- API endpoints

* Add validation hook

* Custom check for the 'definition' field

* Add settings page for custom units

- Table of units
- Create / edit / delete buttons

* Allow "unit" field to be empty

- Not actually required for custom unit definition

* Load custom unit definitions into global registry

* Docs: add core concepts page(s)

* Add some back links

* Update docs

* Add unit test for custom unit conversion

* More unit testing

* remove print statements

* Add missing table rule

docs/docs/concepts/units.md

@@ -0,0 +1,41 @@
+---
+title: Physical Units
+---
+
+## Physical Units
+
+Support for real-world "physical" units of measure is implemented using the [pint](https://pint.readthedocs.io/en/stable/) Python library. This library provides the following core functions:
+
+- Ensures consistent use of real units for your inventory management
+- Convert between compatible units of measure from suppliers
+- Enforce use of compatible units when creating part parameters
+- Enable custom units as required
+
+## Unit Support
+
+Physical units are supported by the following InvenTree subsystems:
+
+### Part
+
+The [unit of measure](../part/part.md#units-of-measure) field for the [Part](../part/part.md) model uses real-world units.
+
+### Supplier Part
+
+The [supplier part](../part/part/#supplier-parts) model uses real-world units to convert between supplier part quantities and internal stock quantities. Unit conversion rules ensure that only compatible unit types can be supplied
+
+### Part Parameter
+
+The [part parameter template](../part/parameter.md#parameter-templates) model can specify units of measure, and part parameters can be specified against these templates with compatible units
+
+## Custom Units
+
+Out of the box, the Pint library provides a wide range of units for use. However, it may not be sufficient for a given application. In such cases, custom units can be easily defined to meet custom requirements.
+
+Custom units can be defined to provide a new physical quantity, link existing units together, or simply provide an alias for an existing unit.
+
+!!! tip "More Info"
+    For further information, refer to the [pint documentation](https://pint.readthedocs.io/en/stable/advanced/defining.html) regarding custom unit definition
+
+### Create Custom Units
+
+To view, edit and create custom units, locate the *Physical Units* tab in the [settings panel](../settings/global.md).

docs/docs/part/parameter.md

@@ -77,13 +77,13 @@ The parametric parts table allows the returned parts to be sorted by particular
 
 ## Parameter Units
 
-The *units* field (which is defined against a [parameter template](#parameter-templates)) defines the base unit of that template. Any parameters which are created against that unit *must* be specified in compatible units. Unit conversion is implemented using the [pint](https://pint.readthedocs.io/en/stable/) Python library. This conversion library is used to perform two main functions:
-
-- Enforce use of compatible units when creating part parameters
-- Perform conversion to the base template unit
+The *units* field (which is defined against a [parameter template](#parameter-templates)) defines the base unit of that template. Any parameters which are created against that unit *must* be specified in compatible units.
 
 The in-built conversion functionality means that parameter values can be input in different dimensions - *as long as the dimension is compatible with the base template units*.
 
+!!! info "Read Mode"
+    Read more about how InvenTree supports [physical units of measure](../concepts/units.md)
+
 ### Incompatible Units
 
 If a part parameter is created with a value which is incompatible with the units specified for the template, it will be rejected:

docs/docs/part/part.md

@@ -81,6 +81,9 @@ By default, all parts are *Active*. Marking a part as inactive means it is not a
 
 Each type of part can define a custom "unit of measure" which is a standardized unit which is used to track quantities for a particular part. By default, the "unit of measure" for each part is blank, which means that each part is tracked in dimensionless quantities of "pieces".
 
+!!! info "Read More"
+    Read more on how InvenTree supports [physical units of measure](../concepts/units.md)
+
 ### Physical Units
 
 It is possible to track parts using physical quantity values, such as *metres* or *litres*. For example, it would make sense to track a "wire" in units of "metres":

docs/mkdocs.yml

@@ -70,15 +70,17 @@ nav:
   - InvenTree:
     - InvenTree: index.md
     - Features: features.md
-    - Release Notes: releases/release_notes.md
     - FAQ: faq.md
-    - Credits: credits.md
-    - Privacy: privacy.md
-    - Terminology: terminology.md
+    - Core Concepts:
+      - Terminology: concepts/terminology.md
+      - Physical Units: concepts/units.md
     - Development:
       - Getting started: develop/starting.md
       - Contributing: develop/contributing.md
       - Devcontainer: develop/devcontainer.md
+    - Credits: credits.md
+    - Privacy: privacy.md
+    - Release Notes: releases/release_notes.md
   - Install:
     - Introduction: start/intro.md
     - Configuration: start/config.md