Improved the pnac ofdm sync block. This is based on a VTC'99 paper by Tufvesson,...

Improved the pnac ofdm sync block. This is based on a VTC'99 paper by Tufvesson, et al. that does a bit more work than the Schmidl and Cox to produce a more identifiable peak for the timing. This seems to work well in the simulation for low frequency errors. The correlation doesn't seem to track well, though. See the comments for more info. Also, the peak detection requires unity amplitude for the threshold detection. So, who wants to make an OFDM AGC?

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@8217 221aa14e-8319-0410-a670-987f0aec2ac5