Support for populations/templates

[jdcpni]

[pgleeson]

The idea is that we could like to (pre) define a node template and then say "I'd like N instances of these", without needing to redefine the behaviour every time. Ideally these can have different parameters, or be customised from the base template.

Note though that "populations" are relatively easy to create when all the entities have similar structure, just using arrays for the state variables. e.g. https://github.com/ModECI/MDF/blob/nml_examples/examples/MDF/RNN/iaf.net.png