Revive and redesign JSON interface

[cortner]

We can use this issue to discuss the revival and redesign of the JSON interface.

CC @CheukHinHoJerry @wcwitt @gelzinyte @iamveronika

[CheukHinHoJerry]

During fitting, would it be useful to write a logging file that output the FULL hyper-parameter dictionary with all the details of the fits for keeping record (even for random seed etc)? I found this pretty useful when working with mace that I can just look at the log and make sure nothing does wrong.

[cortner]

Ok for me to add this.

[cortner]

more to do re polishing but the main ideas are addressed via #231