Implement BiRNN GP Calculation (#1)

[santacml]

This PR implements calculating the BiRNN GP using summation and concatenation on the BiRNN hidden states.

The PR adds the python notebook, sim, and theory files for BiRNN, and also adds a gitignore to ignore some of the cached files.

The plots notebook is not edited to maintain consistency with the paper.

Merged via squash before creating PR.

Co-authored-by: Michael Santacroce (Microsoft Email)