Add possibility to specify workflow migration

ldvspec/lofardata/migrations/0007_workspecification_selected_workflow.py

+# Generated by Django 4.1 on 2022-08-11 09:24
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('lofardata', '0006_atdbprocessingsite_workspecification_processing_site'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='workspecification',
+            name='selected_workflow',
+            field=models.CharField(max_length=500, null=True),
+        ),
+    ]