Fitting overhaul

[alasheen]

The fitting.profile package is fully documented and with unittests.

The next step on this branch is to continue the development of interfaces.beam.analytic_distribution

Remaining todos for later:

• TODO: improve treatment of the baseline
• TODO: improve the implementation of options as dict and **kwargs
• TODO: For all fitting function, add formula and default initial guess for the fit
• TODO: detail options for each function, especially for fitting
• TODO: include the changes from interfaces.beam.analytic_distribution
• TODO: in unittests the precision is set manually atm and should be reviewed