Change item quantity field from PositiveInteger to Decimal

- Allow 'partial' quantity e.g. '0.45kg'
- Need to change some maths functions as Decimal type is pernickity

InvenTree/stock/migrations/0016_auto_20191118_2146.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2.5 on 2019-11-18 21:46
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0015_auto_20190913_1407'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='stockitem',
+            name='quantity',
+            field=models.DecimalField(decimal_places=5, default=1, max_digits=15, validators=[django.core.validators.MinValueValidator(0)]),
+        ),
+    ]

InvenTree/stock/models.py

@@ -323,7 +323,7 @@ class StockItem(models.Model):
     batch = models.CharField(max_length=100, blank=True, null=True,
                              help_text=_('Batch code for this stock item'))
 
-    quantity = models.PositiveIntegerField(validators=[MinValueValidator(0)], default=1)
+    quantity = models.DecimalField(max_digits=15, decimal_places=5, validators=[MinValueValidator(0)], default=1)
 
     updated = models.DateField(auto_now=True, null=True)