Revert to MathJax in Sphinx documentation

Revert to MathJax (from PNG rendering) in `conf.py` Sphinx documentation
file.
Extend `identify_variables()` with optional suffix

Add additional keyword argument to `identify_variables()` in
`fsic.parser.code` to allow for suffix in regular expression e.g. to
retain trailing period index.

Add new test case for new keyword argument.
Update `make_graph()` call to include suffix

Update call to `identify_variables()` in `make_graph()`, to pass
`suffix` keyword argument. This preserves the trailing period index and
is necessary to uniquely identify equation terms.