Add support for specifying neuron models in NESTML. A simple demo is implemented using the "120 cells" point neuron network.

Todo

• Update installation requirements, Docker image, CI, ...
• Check that this still works for larger networks with multiple populations. NEST has the issue (that is being worked on) that module unloading or reloading is not supported. If necessary, this can be worked around by adding GUIDs to each individual module name.
• Support for specifying edges (synapses and synaptic plasticity rules) using NESTML.