NsxFile.getdata doesn't subselect electrodes

[TomDonoghue]

When trying to load a subset of a datafile, with NsxFile.getdata, setting elec_ids to a sub-selection of electrodes doesn't seem to actually change the set of loaded data - the output dictionary a note of the 'selected' electrode IDs, but but the data attribute always seems to contain the full set of electrodes. By quickly scanning the code, it seems that at some point in the recent updates something must have been removed - there is code that accepts and checks the electrode IDs, and adds that to the output, but no code that actually applies the selection.