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

[timburke]

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.

[timburke]

This pull request has now been updated to include support for capturing and checking the I2C output of an 8-bit unit test. This functionality is required for beginning to refactor and update the pic12_executive mib code so that we can ensure a bug-free upgrade. See test_bus_master.as in pic12_executive for example code on how to use the i2c_capture feature.

Also, this does the following:

• Refactors unit test code to allow for 12-bit and 24 bit unit tests to define and respond to different test attributes in the test file header
• Allows unit tests to add additional checks into whether a test is successful (like checkpoints being reached and i2c output matching a desired sequence.

@amcgee This PR should be ready now and is blocking work on refactoring the mib code since I need i2c output validation to feel comfortable modifying the packet structure.

[amcgee]

Merging. I will integrate this with the CI system separately.