Support network input

[mstimberg]

For now, we do not support feeding external input into a network which limits the models we support. Have a look through Brian2s and NeuroML's classes for this purpose and see which ones can be translated easily. I think SpikeGeneratorGroup and PoissonGroup should be straightforward at least.

[pgleeson]

See here: https://github.com/NeuroML/NeuroML2/blob/master/examples/NML2_Inputs.nml for the types of inputs allowed in NML2