improved fdmdv channel simulation function and wrote cmd line util to test it.  getting same BER results from Octave and C demods which is a good sign.  Some of the fdmdv_demod_c plots a bit messed up but BER results correct

git-svn-id: https://svn.code.sf.net/p/freetel/code/codec2-dev@1780 01035d8c-6547-0410-b346-abe4f91aad63