Add 'virtual' part flag

InvenTree/part/migrations/0009_part_virtual.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.2 on 2019-06-18 08:33
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0008_auto_20190618_0042'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='part',
+            name='virtual',
+            field=models.BooleanField(default=False, help_text='Is this a virtual part, such as a software product or license?'),
+        ),
+    ]