Fix multinest timing

This fixes the current time-keeping in multinest so that the new time interval is calculated with respect to the last time the sampling time file was written, not with respect to the time when the run started. This should close #500 (closed).

bilby/core/sampler/pymultinest.py

@@ -271,6 +271,7 @@ class Pymultinest(NestedSampler):
         current_time = time.time()
         new_sampling_time = current_time - self.start_time
         self.total_sampling_time += new_sampling_time
+        self.start_time = current_time
         with open(self.time_file_path, 'w') as time_file:
             time_file.write(str(self.total_sampling_time))