Added PartCategory

- Parent can be null (top-level category)
- Parent can be other PartCategory

InvenTree/part/models.py

@@ -0,0 +1,14 @@
+from __future__ import unicode_literals
+
+from django.db import models
+
+class PartCategory(models.Model):
+    name = models.CharField(max_length=128)
+    description = models.CharField(max_length=512)
+    parent = models.ForeignKey('self', on_delete=models.CASCADE, blank=True, null=True)
+    
+    def __str__(self):
+        if self.parent:
+            return str(self.parent) + "/" + self.name
+        else:
+            return self.name