Add stratified resampling; improve handling of -inf weights

[benlebrun]

This PR adds stratified resampling to smc_standard. The resampling method can be specified by a new resampling_method argument to smc_standard which defaults to multinomial (and thus preserves the past behaviour).

I have also improved the handling of cases in which all particles have -inf weight. Previously our logsumexp method would return nan and raised a warning. I've added a case to logsumexp which returns -inf when all input weights are -inf. We also skip the resampling step (and ess_threshold check) when all particles are -inf.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
llamppl/inference/smc_standard.py	88.46%	3 Missing :warning:
llamppl/util.py	66.66%	1 Missing :warning:

:loudspeaker: Thoughts on this report? Let us know!