Move BGZF to its own document

[zaeleus]

BGZF is currently described in a subsection under the BAM format: § 4.1 "The BGZF compression format". However, BGZF is not (or no longer?) specific to BAM. Given both its general usage and application in other formats (e.g., BCF, CSI, tabix, etc.), the section may be better served out of the SAM specification and in its own document.

[jmarshall]

Yes, this is #70 (comment) and one of these days I will find my draft of this from the time…