Refactor pystepsrc

[dnerini]

This new version maybe is a good opportunity to change the pystepsrc parameter file format. Although the JSON configuration file does the job, it has a few disadvantages. A better, and more readable, alternative is the YAML format that I personally like a lot.

Originally posted by @aperezhortal in #216 (comment)

[dnerini]

I like the idea of the yaml format (and for this, MeteoSwiss maintains a very nice little library that we could adopt: yaconfigobject). Other than the format, we should rethink the scope of the configuration files and how this is used throughout the library.