Break up RIGS models into seperate files

RIGS/models/utils.py

@@ -0,0 +1,9 @@
+def filter_by_pk(filt, query):
+    # try and parse an int
+    try:
+        val = int(query)
+        filt = filt | Q(pk=val)
+    except:  # noqa
+        # not an integer
+        pass
+    return filt