Adding preliminary support for signal names.

More to come, this should probably be included in the simulator_objects
constructors, to help with debugging in general.
Adding test_nonlinear

Filling in support for rate mode, trying to track down
bug in handling of lif bias.
New old_api test (same old test with more neurons)

There is a bug in the current handling of the neuron bias, trying to find it.
Added input, bias, output signals to nonlinearity

Double-storage of bias_signal.value and bias caused incorrect
simulation.
Simulator supports Constant signals without filter

It is now an error to make a filter or transform whose output signal
is a constant. model.filter() and model.transform() check this condition,
it might be more correct to move this check to the constructors of the
respective objects.
tighten up old_api test tolerance
returning test_old_api show flag -> False
Fix tests now that filter dests cannot be Constant