Add Forwards Filtering Backwards Sampling to hmm_lib.py

[korntewin]

Description

Add Forwards Filtering Backwards Sampling to hmm_lib.py (Google Colab Notebook Included)

Issue

#742

Steps

• [x] Add new code following the PEP 8 guidelines.
• [x] Implement Forwards Filtering Backwards Sampling algorithm using vectorized jax.numpy following the CONTRIBUTING.md.
• [x] Sanity test on Google Colab.

Checklist:

• [x] Performed a self-review of the code
• [x] Tested on Google Colab.

Potential problems/Important remarks

I have implemented the Forwards Filtering Backwards Sampling into hmm_lib.py as per this issue #742. Please review and feel free to suggest the code @murphyk @mjsML @gerdm.