Adding rst generator script

[ferdonline]

To integrate with sphinx documentation the ipython notebooks need to be converted to html and have a stub

This PR

Creates a script which uses nbconvert to convert the ipynb files to html and generates the index.rst and $name.rst stub file.

[alexsavulescu]

This script will not be needed anymore here. I've moved it to neuron and generalized it:

• cmake sphinx integration - documentation under single tree nrn#622