TRestDataSetGainMap improvements

[AlvaroEzq]

Some improvements to the TRestDataSetGainMap:

• Naming the segmented spectra TH1F. Better practice to name them appropiately.
• Simplifying some overloads. This avoids explicit casting on parameters of DrawSpectrum, DrawLinearFit and Refit functions when calling them. This simplyfies the procedure of checking and refitting the peaks.
• Adding function UpdateCalibrationFits(). This is useful to write auxiliar code (macros) that help on the refitting of the peaks.
• Updating and improving documentation