WIP: Scatter digraph

This merge includes a basic matrix inverter which uses the interface necessary for network reduction of the scatter graph (the full algorithm hasn't been ported yet).

In addition. components/mirror.py and simulation.py have been annotated with ideas to improve/modify the matrix fill interface to accomodate the network graph interface, as well as make it a bit more maintainable through a visitor pattern + manipulation/query interface.

It also includes setup to use py.test for testing. Highly recommended over notebooks, particularly if fixtures are used to setup output paths for plots. With pytest-watch also installed, the test code runs as you modify code. Highly efficient.