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
Just a cosmetic: would be better to change 'sample' to 'samples_latex[sample]' from latex_labels, and add V+Jets in that dictionary.