Fix STDP access counter logic

[clinssen]

The access counter should not have a multiplicative factor num_transferred_variables, because each synapse only calls get_history__() once on its postsynaptic partner, regardless of how many variables were transferred. The main change of this PR is to remove num_transferred_variables. This requires postsynaptic variables to be read from the postsynaptic spike buffer iterator (start->...) instead of from a postsynaptic getter method (get...(_tr_t)).

Also fixes an issue with initialisation of synaptic weight. @heplesser: your test case was added as an integration test and now passes, but I have changed the spike times, could you double-check?

Fixes #1057.

[heplesser]

This should fix #1057.