Fix the multinest temporary file transfer

Previously, when running with multinest on condor, there wasn't enough time given between the eviction signal and when the job was getting kicked out for the exit sequence to complete successfully. This meant that because of the link between the temporary directory and the proper path, all progress was being lost. This MR introduces a dumper function that copies over the output from the temporary directory to the proper path every time the output is updated. I have also added a method to track the sampling time across restarts. I have tested this both locally and on condor, and it works as expected.

bilby/core/sampler/pymultinest.py

@@ -2,7 +2,7 @@ import importlib
 import os
 import tempfile
 import shutil
-import distutils.dir_util 
+import distutils.dir_util
 import signal
 import time
 import datetime