Add a fix to solve few problems that may led to an invalid XML

[jgalan]

There are other places where > needs to be used, for example inside the globalCut definition, inside TRestAnalysisPlot, the condition value can be something like >10. This could be solved by adding new optional fields greaterThan, lowerThan and equals.

This is done in a similar way inside TRestDataSet.

We also need to fix the command field inside TRestTask so that we can write plot.CombinedCanvas() instead of plot->CombinedCanvas().