Recording ``gsyn_exc`` and ``gsyn_inh``

[neworderofjamie]

This needs to be a special case as GeNN handles each synapse groups input's separately. The best solution is probably to implement this in a new GSynMonitor class in recording.py:

1. In Recorder._record add additional cases for "gsyn_exc" and "gsyn_inh" which add a GSynMonitor - this can maybe build a list of what incoming projections parent.population with correct receptor_type (i.e. "inhibitory" if we're recording "gsyn_inh") (a new list might need adding to Population to track incoming projections)
2. In Recorder._record_vars, for "gsyn_exc" and "gsyn_inh", loop through the list of projections previously obtained by the GSynMonitor and copy the data from the GPU using model.pull_var_from_device(SYNAPSE_GROUP_NAME, "inSyn")
3. In the GSynMonitor implement init_data_view using SynapseGroup._assign_external_pointer to get access to each synapse group's "inSyn" variable
4. Override GSynMonitor.__call__ to sum these views.

This is infact more general. For example the GIF neuron allows recording of two adaption variables which are the sum of multiple actual state variables.