Tidying up some doxygen niggles

1. Use fewer columns.  Horizontal scrolling on anything but a Mac is a disaster
2. Use short names for the html doxygen pages.  These names are not stable,
   but GitHub doesn't like the long names because a lot of them lead with an
   underscore
Merge pull request #225 from dmcdougall/fix-doxygen

Tidying up some doxygen niggles