Improve 8-bit unit testing and update pymomo.simulator to support gpsim

This PR does many things:

1. Adds a patch for gpsim to add support for the 16lf1847 processor
2. Update the unit tests for the pic12_executive to run on the newly added processor
3. Adds support for gpsim to pymomo.simulator so that it can be used from the same interface as sim30.
4. Updates the 8-bit unit testing framework to support some more features and adds initial documentation for how to write and use unit tests.