Support tagging and accessible PDFs

[dcpurton]

Both refactoring and testing is required to properly support accessible PDFs

• Change list environments out for new block environments (otherwise everything is tagged as lists)
• Change method of measuring the length of the final line to use xpos rather than display maths hack (otherwise there are maths tags included)
• Headings need to be appropriatly tagged
• Consider tagging structure of various elements
• Test