Add a `more_tearDown` method to MpiTestCase.
Improve the docstring for `temp_filepath`.
WIP: Modify LocalArray `save` and `load` APIs..

to take various things instead of filename prefixes for a save target.

I've taken some code from numpy to allow these functions to take

1) an open file object,
2) a full filename string ('blahblah.dnpy'), or
3) (for save only) a filename string without an extension ('blahblah'), wherein save will now append '.dnpy'.

This is WIP because the client code hasn't been updated yet.
Make the new API work on the client-side.

Now, the code to specify per-process filenames is client-side.

No tests yet for the 'different-name-per-engine' functionality.
Fix client io, improve and expand tests.