Removed redundant IDMixin.__getattribute__ call from IDMixin.__getattr__ 
method, as __getattr__ is only called if __getattribute__ fails in any
case.

Also, slightly reordered the common.Population.__init__ and the setting
of Population.first_id/last_id so that it is possible to take a view of
the population while the cells are being built instead of getting stuck
in an infinite recursive loop (although it only an issue when
debugging).
removed commented out code
Merge branch 'easier_debugging_of_neuron_build_cells'