Add DataType::isToplike() #166

Currently the DataType is a pure enum which is really nice when you want to loop over things while giving it a name.

I would like to make it into a struct/class while maintaining the functionality to compare it to the Enum.
Hence I suggest to add the function bool DataType::isToplike() const using the same methodology as Ryan did with his FileType (albeit with comparison operator):
https://github.com/BristolImageAnalysis/SensorAnalysisToolkit/blob/master/modules/Plotting/Histograms/include/stkImageHistogram.h