Add userdict in PyNEST to let users store custom data with the kernel

[babsey]

This PR adds the variable userdict in nest.

This feature is useful to store and transfer metadata via NEST Server
This can be applied for NEST_Desktop-NRP and NEST_Desktop / CoSim-NEST-TVB usecases.

[jougs]

@babsey: I think this should also be documented somewhere. Probably by just adding a docstring to the variable. The documentation should also contain an example of how this would be used.

@heplesser: do you think userdict is a good name? Or should this be rather userdata or customdata or something similar?

[babsey]

That's pity that this PR is rejected.
I would like to see solid reasons for the rejection in a comment before I close this PR.

From my perspective, it was the easiest solution to find the communication between
different tools which are both using nest.

I want to highlight the concept of this PR and describe these two usecases:
NEST Desktop only prepares simulation with constructed network.
Both CoSim or NRP wants to know which nodes are relevant for the transfer function.

In NRP we want to get spikes from 8 nodes for the robot brain.
Our temporary solution is to take only the first 8 nodes (nest.GetNodes()[:8])
In this case we are restricted to the predefined network (fixed size) which is not a proper solution.

In CoSim, we want to record spikes for the transfer to other tools like TVB or Arbor.
Here, we had 2 options:

• Option 1: Use template for nest_adapter.py with an additonal server instance (CoSim Server).
  However we have some troubles to work with this option.
• Option 2: Use nest.userdict to get an array of relevant nodes. This was our best solution.

[babsey]

I apologize. I misunderstood during the VC meeting.

I will discuss with CoSim and NRP people if they also acknowledge this changes.
We will come back to this PR with better generative description.

[jougs]

@heplesser: can you please quickly have a look at this? This feature is required externally and given the smallness of the changeset I think we should be able to quickly decide...