Added prerequisites to add tests and db convenience

- api/convenience will hold convenience functions for database lookup
  and data grouping
- pt_map/model_test_fields holds the data from the GTFS reference
  relevant to model development
- pt_map/test_data holds example data to be used to test models

transport_accessibility/api/convenience.py

@@ -0,0 +1,4 @@
+from pt_map.models import *
+import osmnx
+
+