Convert some more fields to decimal

- purchase order line item quantity
- purchase order line item received

InvenTree/order/migrations/0013_auto_20191118_2323.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2.5 on 2019-11-18 23:23
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('order', '0012_auto_20190617_1943'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='purchaseorderlineitem',
+            name='quantity',
+            field=models.DecimalField(decimal_places=5, default=1, help_text='Item quantity', max_digits=15, validators=[django.core.validators.MinValueValidator(0)]),
+        ),
+    ]