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/part/migrations/0024_auto_20191118_2139.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2.5 on 2019-11-18 21:39
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0023_auto_20190913_1401'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='bomitem',
+            name='quantity',
+            field=models.DecimalField(decimal_places=5, default=1.0, help_text='BOM quantity for this BOM item', max_digits=15, validators=[django.core.validators.MinValueValidator(0)]),
+        ),
+    ]