Generate new neuron model in VERTEX from equivalent in LEMS

[pgleeson]

For example generate a Matlab script like this:
https://github.com/richardtomsett/vertexsimulator/blob/master/vertex/vertex_simulate/models/neurons/NeuronModel_adex.m

from a LEMS description like this:
https://github.com/NeuroML/NeuroML2/blob/master/NeuroML2CoreTypes/Cells.xml#L1023