Provide doxygen infrastructure to turn comments into developer documentation

[jdasmith]

Refer to https://www.stack.nl/~dimitri/doxygen/manual/starting.html https://www.stack.nl/~dimitri/doxygen/manual/doxygen_usage.html https://www.stack.nl/~dimitri/doxygen/manual/docblocks.html#fortranblocks

So - probably use doxywizard to create a conf file, then test it out! But need a bit more content in the .f90 files first!

[mightylorenzo]

I can gen the documentation when working with the 'source' Puffin, but after make and make install, it no longer works. We'll need to point the generated fortran.dxg file to the .f90 files somehow!