Update GIDB data model and REST API to support multiple models (as needed)

[selimnairb]

GIDB data model and REST API may need to be updated to allow for one or more models to be applied to a GI scenario.

[selimnairb]

To support IUHM we need to add:

GI Elements:

• Soil depth
• Ponding depth

To support multiple models we need to add:

GI Template:

• gi_type (a controlled vocabulary of GI types: tree, rain garden, green roof)

To support multiple models in general, we need to add:

Model types:

• Name
• Code (something like the name, but without spaces)

Watershed model:

• Foreign key to watershed
• Foreign key to model type
• Model URL