Use MaskStorage for subsamples cuts

Created by: mmarchegiani

The present PR implements the use of the MaskStorage object to store the subsamples masks. Now the configuration to define the subsamples follows the same scheme as the categories. For each sample, one can specify the subsamples as a dictionary, a StandardSelection or a MultiCut. The Configurator module will take care of converting the dictionary into a StandardSelection. The dictionary of subsamples returned by the Configurator is effectively a dictionary of dictionaries whose values are StandardSelection and MultiCut objects. The new implementation has been tested on a configuration with a categorization per object and a definition of subsamples per object. It was checked explicitly that the subsamples are properly defined when they are defined per object.