Random access to blocked XZ format (BXZF)

I've written about random access to the blocked GZIP variant BGZF used in BAM, and looked at random access to BZIP2, but here I'm looking at XZ files which are based on LZMA compression. This was prompted by the release of Python 3.3 which includes the lzma module to support XZ files, which I then back-ported to offer lzma for Python 2.6 or later. Over the Christmas / New Year break I extended this to handle Blocked XZ Format (BXZF for short), so publishing this post is a bit overdue.