Gr.flow_graph and friends are gone

We’ve removed gr.flow_graph, gr.hier_block from the trunk and
converted the bulk of everything to use the new gr.top_block and
gr.hier_block2 interfaces. Thanks go to Johnathan for his yeoman
service in converting the bulk of the examples and QA code. Not to
mention writing the new stuff in the first place.

This clears the way for the “thread-per-block” scheduler which will
allow GNU Radio to make good use of SMP and SMT machines, and is
another step towards “pure C++” applications in GNU Radio.