Add support for Synapses to Synapses

[denisalevi]

In brian2 one can create Synapses object which target Synapses objects. We currently get an

NotImplementedError: Cannot retrieve the values of state variables in standalone code before the simulation has been run.

error. This fails a bunch of test suite tests.
The reason is, that we need the IDs of the target group of a SynapticPathway in order to create our connectivity matrix (which is split by groups of post IDs). To get those, we need access to state variables of the SynapticPathway objects target group, which is a Synapse in the case of synapses to synapses connections. And we can't access Synapse variables before synapse creation and our current implementation fills those target group variables from python side (before the network is run), which gives us aboves error. I've tried to fix this but it was not straight forward and I'm moving on to more urgent tasks. Check the synapses_to_synapses_support branch for my first try. It needs some rearranging, such that the SynapticPathway classes (in cpp) target_start/target_end variables are some variables that are set from cpp side after synapse generation templates are run.

Maybe we also don't need my additional parameters in the SynapticPathway class (see commits below) and can just work with the owner.target.start variable in the synapses_initialise_template.

[denisalevi]

Maybe relevant? brian-team/brian2@6e8acd1b

[denisalevi]

Here are two PRs I came across related to Brian2 changes for synapse to synapse connections: brian-team/brian2#715 and brian-team/brian2#854.

[denisalevi]

This was fixed in PR #186.

Closing.