New class TRestDataSetPlot to perform plots over datasets

[juanangp]

New class TRestDataSetPlot implemented which is meant to perform plots over a TRestDataSet and it is meant to replace TRestAnalysisPlot.

Summary of the changes:

• TRestDataSetPlot metadata similar to TRestAnalysisPlot with with some minor modifications.
• Cuts implemented using generic TRestCut
• New method implemented to add several cuts to the same panel, histo or global cut
• Renaming SetDataSet to SetDataFrame in TRestDataSet since the first one is misleading.
• Moving quantity on TRestDataSet from Double_t to string since it is stored as string in the TRestRun metadata
• Bug fix on REST_StringHelper::GetChar function since it was causing segfault

[jgalan]

@juanangp it would be good to have a RML example. Inside the directory https://github.com/rest-for-physics/framework/tree/master/examples

[juanangp]

@juanangp it would be good to have a RML example. Inside the directory https://github.com/rest-for-physics/framework/tree/master/examples

Done

[jgalan]

@juanangp Should be merged already?

[juanangp]

@juanangp Should be merged already?

I would like to do further changes since I think this PR might have some conflicts with the one that you just merged #432

I hope will be ready later in the afternoon.