Read fit information from `json` file

Created by: scarlehoff

Doing this will break compatibility with nnfit but I believe it's about the right time to do this. Once we move to the new theory or the new commondata it won't make sense to use newer updated versions of vp with old fits.

This will also simplify @Radonirinaunimi efforts for the simultaneous fits (since it is better to parse and write info from some json file rather than adding extra lines in a particular order to the .fitinfo (and other) files).

Since we are doing yaml for the data it might make sense to convert the json into yaml (which is trivial because it's a reasonably simple dictionary so keeping json/yaml compatibility would be easy).

I've marked this as NNDPF4.1 since I think it is reasonably to expect all compatibility with nnfit to be dropped by then.