Use `<fitname>.json` by default and deprecate the `.fitinfo` file.

Created by: scarlehoff

Closes #1543 (closed)

From now on the fits won't write all the different files (.fitinfo, .version, .preproc, etc) but everything will be part of a .json file.

I've left a fallback to the .fitinfo in the reader since we want to be able to compare with old fits. I've removed a few functions that were not used by anything else and I've added a check to ensure that the same version was used to run all replicas of a given fit. The check was already there in the past but for some reason I put it at the end instead of at the beginning. I guess I was younger then ¯_ (ツ)_/¯