Bug fix to encoding the 'd' (double) aux data type.  We've never tested
this as it's not even in the SAM spec, but I added it anyway as
samtools seems to include it.


git-svn-id: svn://svn.code.sf.net/p/staden/code/io_lib@3668 ba6f2710-ca9f-427c-89d3-5bf716dbc8e1