simulation of FEC applied to 450 bit/s codec.  Estimate of correctable errors with a good code makes it useful to about 0 dB, but more work to be done, e.g. test with a real code, interleaving

git-svn-id: https://svn.code.sf.net/p/freetel/code/codec2-dev@1878 01035d8c-6547-0410-b346-abe4f91aad63