Add information form filtering.

[gileshd]

Filtering

ssm_jax/lgssm/info_inference.py contains code for kalman filtering in information form.

The implementation more-or-less mirrors the filtering in lgssm.inference with _predict() and _condition_on() changed to accept and return precision (Lambda) and precision-weighted mean (eta).

Tests

ssm_jax/lgssm_info_inference.py contains tests which compare the output of lgssm.info_inference.lgssm_info_filter to lgssm.inference.lgssm_filter. There are currently two tests:

• test_info_kalman_filter - simple test with stationary system.
• test_info_kf_linreg - uses the example in lgssm/demos/kf_linreg.py to test non-stationary parameters.

[murphyk]

beautiful :)