Config changes

Replaced the configuration files (3) with a class (config.XSectionConfig) that set's the right values depending on the centre of mass energy.

This also includes synchronisation of file names (where possible), whcih for 7 TeV changed from
```muon_QCD_MC_file = path_to_files + 'central/QCD_Pt-20_MuEnrichedPt-15' + middle + '.root'```
to
```muon_QCD_MC_file = path_to_files + 'central/QCD_Muon' + middle + '.root'```
as it is for the 8 TeV.

I've included some unit tests, but I might have missed something. I ran x-section scripts 01-05 and all tests (all work).