Handle epJSON format

[thomascerbelaud]

Since the IDF format is soon to be deprecated in favor of a more adapted format (epJSON) (informatically speaking), it would be good to account for epJSON files officially:

• Parse epJSON and save Epm to epJSON: I am not sure are compatible this with Epm.to_json_data and Epm.from_json_data methods. We may want to load the file as an IDF and use the command line to do the conversion for now. Note that the epJSON format is supposed to be case sensitive.
• Simulate epJSON files: once it is loaded as an Epm, this is more straightforward, so I guess the only thing would be to add the -c option when running the process to generate an opyplus.epJSON file in the simulation folder. Or we may pass directly the epJSON file to the process instead of the IDF.