Ticket #2 (new task)

Description

Currently, a standard model combines both a membrane mechanism and a synaptic mechanism. If there are n of the former and m of the latter, we have to create n x m standard models (even more if there could be more than one synapse type per neuron). It would be cleaner to have standard membrane models and standard synaptic models that could be combined in user code to build arbitrary models. Only n + m models then have to be created/maintained.