TRestDataSetGainMap improvements (#548)

* fix bug on the automatic naming of the output calibrated dataset

* fix error when not defining the module definition cut by setting default to '1'

* fix isDataSet crash when input file didn't exist

* add tool function GetObservablesInString

* allow multiple TRestRun input by generating TRestDataSet on the fly

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* improve documentation

* add macro for easy refitting

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* fix precommit messing with class documentation

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: AlvaroEzq