Early work on main `while` loop

Early work on main `while` loop to reorder equations according to the
in-degree of the nodes, in a graph representation of the equation
system.
Initial work on node handling for in-degree zero

Initial work on core `while` loop, to calculate in-degree of nodes and
identify those where in-degree is zero, for deletion. Still to add code
to pop into new equation order.
Update Sphinx configuration

Update Sphinx `conf.py` to use PNG mathematics rendering, rather than
MathJax.

Update Sphinx `conf.py` to import FSIC version number from package
installation.