Izhikevich & HH tutorial related updates

[pgleeson]

• Add more documentation to schema for a, b c d thresh etc. (have started this locally)
• Add a cell with print(nml_doc.summary()) in notebook(s)
• Use spiking input in network example: https://docs.neuroml.org/Userdocs/Schemas/Inputs.html#poissonfiringsynapse

And for HH:

• add plots of m, h, n - important for understanding innards of model