Add test for NESTML code generation for NEST Simulator

[clinssen]

Add support and an example for NEST Simulator models generated through NESTML.

Work in progress.

Open questions:

• Where to put the (one or more) example NESTML models (.nestml model files)?
• How does the line return type.__new__(cls, name, bases, dct) work? It errors out currently with "TypeError: type.__init__() takes 1 or 3 arguments".
• (How) should this be added to CI?
• Test case with neuron + synapse should be added. Is there a similar facility as NativeCellType, but for synapses?

[apdavison]

@clinssen I realized after returning from Heidelberg that I worked on this last summer, although I have only a vague memory of doing so!

master...apdavison:PyNN:nestml

I think your approach with ast_compilation_unit = ModelParser.parse_file(nestml_file_name) is an improvement on what is in that branch.

We should discuss how to proceed. The remaining steps are to add support for synapses (based on pyNN.nest.synapses.native_synapse_type()) and then to add some tests for CI.

[clinssen]

Closed in favour of #825.