Runtime of PopulationView.id_to_index() has > n^2 scaling with number of cell parameters

[Christian Mauch]

PopulationTest.test_scaling_cellparams_id_to_index from #691 shows that PopulationView.id_to_index() scales > n^2.
relevant line a58cc51#diff-fb732f5879c60a0dc39fffe1a1bb8fbc45c3299c0197baec8e942cf69b871df5R110
The line of code where most time is spent is

PyNN/pyNN/common/populations.py

Line 881 in f740854

 result = numpy.where(self.all_cells == id)[0] 

if IDMixin is not assignend its Population as parent this scaling is not observed
As Population.id_to_index()'s implementation does not use numpy.where() it also does not scale with number of cell parameters