Subset operations for tomography objects

[fede-pe]

The comparison of (some) tomography objects should be done by _tsId field (now is simple id as it is done in SPA). This should be detected for tomography objects, some i can think of are tilt-series, landmark models and tomograms. It is enough to consider this in the subset protocol (protocols_sets line 595 and 596) when constructing hte set of id's (inputFullSet.getIdSet()).

This methods construct a set of the field column id

def getIdSet(self):
""" Return a Python set object containing all ids. """
return set(self.getUniqueValues('id'))

and for this case we want to call the method as:

set(self.getUniqueValues('_tsId'))

The tricky part is how to handle for which object we want this behaviour