Changes:

1. Implemented GibbsStep and GibbsKernel classes
2. Modified mcmc.sample function to use Gibbs sampler
3. Amended bugs in event_time_mh.py and occult_proposal.py (edge cases where tf.gathers
were overshooting the bounds of the data stuctures, not apparent on a GPU but raised on
CPU).

Changes:

1. Added occult Metropolis Hastings update.
2. Factored out Categorical2 distribution for use by both event time move and occults.
3. Refactored mcmc.py script for HDF5 output purposes
4. Apply compression to HDF5 output file.