Code to implement the DEMULTIPLEX codec.

It's largely pointless though as the intended use case (16-bit 454
and IonTorrent flowgrams) can be stored via rANS O(1) codec just as
well:

method comp.    Size        Enc(s)  Dec(s)
BAL    gzip     1602114669  2m23    0m26
DEMULT gzip     1429374986  1m48    0m40

BAL    gz+rANS  1332728825  1m35    0m30
DEMULT gz+rANS  1344441769  1m17    0m36



git-svn-id: svn://svn.code.sf.net/p/staden/code/io_lib@3692 ba6f2710-ca9f-427c-89d3-5bf716dbc8e1