Update reweighting for nessai

Update the reweighting code to avoid using result.meta_data["statistics] since only some samplers report these and instead estimate the time per likelihood call.

Also, include nessai in the list of samples for which the nested samples can be reweighted.

I've tried to keep the changes as simple as possible but I can make further changes if we want, for example, to be able to configure the number of likelihood evaluations used to time the likelihood.

Results

• Run with nessai and calibration reweighting: https://ldas-jobs.ligo.caltech.edu/~michael.williams/bilby-dev/bilby-pipe/calibration-reweighting/outdir_cal_reweight_updated_timing_A/
• Run with nessai and reweight the nested samples: https://ldas-jobs.ligo.caltech.edu/~michael.williams/bilby-dev/bilby-pipe/calibration-reweighting/outdir_cal_reweight_updated_reweight_nest/ (in this run the checkpointing interval was hardcoded rather than being estimated based on the time per likelihood call).