switched to non-standard synapse type for new NEST test
Hopefully avoided problem with automated Travis CI CloneConnector tests
that previously required MPI
wrote electrical synapse test, and added source_section.gap to the list
of acceptable receptor types for a HH_cond_exp neuron type.
Changed from list connector to allow 3 columned data if synapse does not
support delay parameter (eg. Electrical synapses).
Wrote test for electrical synapses
Merge branch 'master' into toms_devel