TRestDataSetGainMap improvements

[AlvaroEzq]

Fix some bugs of TRestDataSetGainMap:

• When moduleDefinitionCut was not set, TRestDataSetGainMap::CalibrateDataSet failed. Now, set the moduleDefinitionCut to 1 (all events accepted) and also check if its value is empty to set it back to 1.
• Automatic naming of output dataset in TRestDataSetGainMap::CalibrateDataSet was wrong. The . before the root extension was missing.
• Fix documentation

Add new feature:

• Now it accepts as dataSetFileName a run file or a file pattern containing several run files. In this new case, it generates the dataset on the fly to get the spectrums needed.
• Add the REST_RefitGainMap macro for a better experience when refitting the peaks manually.

Other changes:

• Add new function TRestTools:GetObservablesInString
• Fix TRestTools:isDataSet to return false if the input file doesn't exist (before it used to crash)