add debug_remote_on_single_rank (#498)

* Move management of current debuggers into Debugger class

* Move term_size heuristic to RemoteDebugger constructor, document better (gh-485)

* Fix post-mortem debugging for Py3

* Remove unexplained stack-eating loop from post_mortem

* add debug_remote_on_single_rank

* flake8

* better doc

* a bit more text

* add an mpi4py example

* allow arguments to func, better doc, simplify

* clarify comment

* more doc

* clarify comment

* add missing args

Co-authored-by: Andreas Kloeckner <inform@tiker.net>