For purposes of reorganising how CRAM codecs are dealt with.
We wish to dynamically choose between internal CORE codecs and
external block codecs based on data content.

Ideally we'll have as many external blocks as we can without
sacrificing file size (it may even improve it) as this gives us more
fine grained control when decoding as to which data series we desire.


git-svn-id: svn://svn.code.sf.net/p/staden/code/io_lib@3605 ba6f2710-ca9f-427c-89d3-5bf716dbc8e1