Model JSON format

[dbhart]

This PR adds full to_dict and from_dict functionality to the WaterNetworkModel. Additionally, this functionality is tested both using just dictionaries but also passing the dictionary in to and out of a JSON formatted file. Any format which can store a dictionary and then produce a dictionary when read in should also work (pickle, yaml, hdf5, etc.).

The to_dict() and write_json(f) functions are available on a WaterNetworkModel object.

The from_dict(d) and read_json(f) function is a class method on the WaterNetworkModel class and are aliased as standalone functions in the wntr.network module.

[coveralls]

Coverage increased (+0.7%) to 84.052% when pulling 1d2dff4 on dbhart:json-model into 7d2d59a on USEPA:main.

[dbhart]

Additional changes: we have never used the SourceRegistry, so I removed it. I also removed the commented-out "to_string" function on the registries.

[dbhart]

@kaklise the code files we went over should be marked as "viewed". The documentation files should still pop up right away when you view the PR "Files changed". Assuming this works the way I think it should.

[dbhart]

Well, shoot. I will go in and reverse that change.

[dbhart]

Created a wntr/network/io.py that now houses the main code for reading/creating/appending to/writing WaterNetworkModel files. The WaterNetworkModel class now calls the functions in this module instead of containing the code to do these IO operations.