Multi variable fitting 

Implementation of multi-variable fit
 - [x] extend Minuit to allow additional variables
 - [x] add helper functions to recast fit-variables into the fit range
 - [x] change fitting script
    - [x] add fit_variables parameter
    - [x] load additional fit_variables
    - [x] store templates for additional variables
 - [x] add scripts to run the fit checks
There are many things done in parallel in this function:
 - getting the normalisation + error
 - creating the template`hist_to_value_error_tuplelist` 

`hist_to_value_error_tuplelist` does only give you the bins and their errors.