Add 'batch' field to BuildOutput object

This 'batch' output will be copied to any parts produced as part of this build

InvenTree/build/migrations/0005_buildoutput_batch.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.12 on 2018-04-17 08:29
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('build', '0004_auto_20180417_0657'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='buildoutput',
+            name='batch',
+            field=models.CharField(blank=True, help_text='Batch code for this build output', max_length=100),
+        ),
+    ]