Improve error messaging for the various no wastewater cases

[kaitejohnson]

Currently if the user passes wastewater data and specifies include_ww = 0, they get this warning:

No wastewater data was passed to the model.
The model will default to fitting only to the count data

Instead, we should have different messaging for each case e.g.

• if no wastewater passed in but include_ww =1 -> warning
• if ww data passed in but include_ww = 0 -> message
• if no wastewater passed in and include_ww = 0-> nothing, both are consistent