This has been forked from CRAMv3.tex c831aba as given this is a major
release we don't wish to keep markup for both versions in the same
document.

This includes a massive reorganisation of the document, which is long
overdue.

It also now replaces EXTERNAL with a variety of type specific codecs
for signed, unsigned and bytes; ditches HUFFMAN (along with all other
bit codecs) and adds CONST codecs instead; removes the CORE block;
removes "external" for block descriptions, instead favouring data
blocks as a way to distinguish from header / structure containing
blocks.

As yet, nothing implements this variant of V4.0! However it's far
cleaner to describe and hopefully far simpler to implement due to
the aggressive culling of largely unused features.

- BYTE is now the same ID as the old EXTERNAL, given it's identical.
  But we refer to it as BYTE for clarity sake.

- Removed sint vs uint in data series types as it's no longer
  necessary to bake this into the specification.

- Moved the Data Series format text to later on and merged with the
  other tables, so the concepts and formats are introduced at the same
  point.

See samtools#747 for CRAM 3.x issue.