Annotate and explain current channel models in NML files

[gsarma]

The models folder in ChannelWorm2 currently has NML files for 4 ion channels. It might be helpful for newcomers to have the structure of these models carefully spelled out. For instance, we could make PDFs of the NeuroML source code and annotate the sections explaining how they correspond to the mathematical ion channel models.

I personally would find it helpful to see the equations spelled out explicitly. See also issue #1.