Refactor gr-howto-write-a-block such that it no longer installs into

Refactor gr-howto-write-a-block such that it no longer installs into
the gnuradio python namespace, but rather in the howto namespace.

This allows it to be installed under a separate --prefix in the common
case where the user doing the "out of tree build" is going to be
installing their new and/or modified code somewhere other than the
system installation directories.

Any code that uses these blocks should now import them using:

  import howto

instead of

  from gnuradio import howto