documentation

There are now three targets for documentation:
`doc_doxygen` --- this runs doxygen on the CPP source code
`doc_html`    --- this runs sphinx on the python code, and it uses the
                  available doxygen xml output to generate
                  sphinx_html/...
`doc_latex`   --- this runs sphinx on the python code, and it uses the
                  available doxygen xml output to generate
                  sphinx_latex/...
I can't get it to a point where I can run just "make doc_latex", I need
separate runs of pdflatex, but I guess that's negligible.