This module was only used to assist testing, but wasn't actually
available in a test environment where no copies of nilmrun have been
installed.  Just code the contents directly into the filters that the
tests are running.

Multiprocessing and Apache's mod_wsgi don't play nicely.  Switch to
manually managing processes via subprocess.Popen etc instead.  When
running arbitrary code, we write it to an external file, and running
functions directly is no longer supported.

Now an entire process tree is killed in /process/remove, as long as
each child hasn't changed its process group.

The multiprocessing environment is really finicky.
I'm seeing deadlocks in the process at exit, that are probably related
to tracebacks being printed and still getting redirected to the
logfile or something.