[Discuss-gnuradio] scopesink / fftsink fixes

[Eric Blossom]

Thanks to the troubleshooting efforts of Jon Jacky, we've now got
scope and fft sinks that work reliably on OS/X.  [There was a problem
with os.read returning a short read count.]

I've also added a 

  gr.throttle(itemsize, sample_rate)

block that limits the average sample rate through the block to
sample_rate samples/sec.  This can come in handy where there is no i/o
device limiting the CPU consumption of GNU Radio.  I've added this
block to the scopesink and fftsink demos so they are now *much* more
responsive. 

To get this stuff you'll need to update and build gnuradio-core and
gr-wxgui from CVS.  [I've noticed a dependency problem with swig .i
files.  Work around: After updating gnuradio-core, cd
gnuradio-core/src/lib/swig, make clean, then build as usual.]

Eric