n3fit - `fit` in `fit.py` should be split into seperate functions

Created by: wilsonmr

As I said in https://github.com/NNPDF/nnpdf/pull/516#issuecomment-514598840 I think fit should actually be several functions:

I think we can all agree on some of these functions:

• initialised seeds
• loading data
• loading positivity data
• fitting

I personally also think that hyperoptimization and fitting should be two seperate functions given that they both produce quite different things, and that the n3fit runcards could then call whichever of these two actions was necessary, since they seem like mutual exclusive tasks. But probably there is more discussion to have on this point.

If all of these functions are in the same file, which is an n3fit module (as far as the App is concerned) then they will benefit from the reportengine wizardry of taking other provider functions as function arguments which get automatically handled by the resource builder