Sampling time is incorrect after checkpointing

The sampling time reported in the bilby result file is incorrect for nessai if the run has been checkpointed and resumed. Instead of reporting the total time, it reports the time since the sampler was resumed. This may affect other samplers, but dynesty and bilby_mcmc both have workarounds and correctly record the time

Note: nessai stores the sampling time in its own result object and this time does accurately track the time after resuming.