Merged changeset r10817:10858 from jcorgan/cpphier into trunk. Adds standard place for C++ hierarchical blocks for gnuradio-core, and new gr.channel_model block from Tom Rondeau.

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@10859 221aa14e-8319-0410-a670-987f0aec2ac5