Resolve sampling_time persistence between restarts for dynesty

This stores the sampling time in the resume file for dynesty and loads it allowing the sampling time for jobs which get restarted to reflect the total sampling time. Also defines the general mechanism for any other samper to act similarly.

Partially resolves #378 (closed)

bilby/core/sampler/__init__.py

@@ -163,15 +163,18 @@ def run_sampler(likelihood, priors=None, label='label', outdir='outdir',
         return sampler.cached_result
 
     start_time = datetime.datetime.now()
-
     if command_line_args.bilby_test_mode:
         result = sampler._run_test()
     else:
         result = sampler.run_sampler()
-
     end_time = datetime.datetime.now()
-    result.sampling_time = (end_time - start_time).total_seconds()
-    logger.info('Sampling time: {}'.format(end_time - start_time))
+
+    # Some samplers calculate the sampling time internally
+    if result.sampling_time is None:
+        result.sampling_time = end_time - start_time
+    logger.info('Sampling time: {}'.format(result.sampling_time))
+    # Convert sampling time into seconds
+    result.sampling_time = result.sampling_time.total_seconds()
 
     if sampler.use_ratio:
         result.log_noise_evidence = likelihood.noise_log_likelihood()