Update core/model.py to core/base_model.py

[davidorme]

Is your feature request related to a problem? Please describe.

We've renamed the actual models (soil/model.py -> soil/soil_model.py) to make the file names all less confusing, but the BaseModel ABC still lives in core/model.py. As discussed in PR #176 , we think this is better explicitly named as core/base_model.py.

Describe the solution you'd like

• Change the name of the file
• Update all the doc links (probably just a search and replace of core.model to core.base_model).