make multinest temporary files optional and work with MPI/resuming

This wound up being a bigger change than I expected, but I've tested that this now

• allows users to turn off the temporary directory usage
• correctly returns the files to the expected directory when interrupted/finished
• clears up the temporary directory when interrupted/finished
• disables the temporary directory usage when running with MPI

I've generally used shutil.move rather than copying and removing as I had all kinds of issues with directories properly being removed.

The other kind of subtle thing is that pymultinest already has a signal handling function which can only be accessed by using importlib rather than a standard import.

Closes #485 (closed)