[Discuss-gnuradio] fftsink and scopesink refactored

[Eric Blossom]

FYI,

fftsink and scopesink were just refactored to use messages and message
queues to communicate between the C++ and Python side.  This removes
our dependency on pipes, but more importantly, the C++ side will now
never block trying to send data to the GUI.

There are also a couple generally useful new blocks:

  gr.message_source
  gr.message_sink

The message source receives data from messages sent to it and writes
it to an output stream.

The message sink reads data from its input stream, generates messages
and inserts them into the provided message queue.

These are currently useful for getting variable length data in and out
of the C++ world.  They currently work within the same address space,
but with a bit of work, could be made to work across address spaces.

Eric