acquisition data subsets

[evgueni-ovtchinnikov]

Design an AcquisitionData.subset() method that returns an AcquisitionData object containing a subset of data specified by the argument of subset selector type, e.g (MR case):

% specify the subset:
selected = AcquisitionDataSubsetSelector()
selected.slices = range(1, 8, 2); # slices 1, 3, 5, 7 selected
selected.repetition = 0 # first repetition selected
% all other dimensions to be the same as in acq_data

sub_acq_data = acq_data.subset(selected) # new AcquisitionData object, can be used as a template, forward-projected etc.
sub_acq_array = sub_acq_data.as_array() # returns array containing selected data