TRestDataSetGainMap improvements and bug fixes

[AlvaroEzq]

Bugs fixing:

• TRestDataSetGainMap::Module::DrawSpectrum and DrawLinearFit didnt associate properly the subpad canvas to the corresponding spectrum/linearfit.
• TRestDataSetGainMap::Module::DrawGainMap had fNumberOfSegmentsX and fNumberOfSegmentsY swapped.

Improvements:

• Description for class attributes and cleaning header comments
• Add cuts to apply to the input calibration TRestDataSet. Printing of the cut was implemented here because the TRestCut::PrintMetadata() does not show the cuts (maybe this should be addressed there instead...)
• Possibility to exclude columns in TRestDatasetGainMap::CalibrateDataSet to exported TRestDataSet
• Add whole module spectrum and calibration. For this, two private methods were added to avoid duplicated code.