16<->8 kHz sample rate conversion filters

git-svn-id: https://svn.code.sf.net/p/freetel/code/codec2-dev@1755 01035d8c-6547-0410-b346-abe4f91aad63
modified DAC driver to be two channels for SM1000, not tested yet

git-svn-id: https://svn.code.sf.net/p/freetel/code/codec2-dev@1756 01035d8c-6547-0410-b346-abe4f91aad63