Octave sim doing filtering on decimated signal, but down conversion etc still at rate M.  However this may be fast enough

git-svn-id: https://svn.code.sf.net/p/freetel/code/codec2-dev@1774 01035d8c-6547-0410-b346-abe4f91aad63
refactoring simulation into functions, about to start C port

git-svn-id: https://svn.code.sf.net/p/freetel/code/codec2-dev@1775 01035d8c-6547-0410-b346-abe4f91aad63