Bring Doxygen comment blocks up to spec.

[mo-mglover]

Most functions have a placeholder comment block, readable by doxygen. They - for the most part - only contain @brief tags. There is no explicit description of arguments yet.

Moreover, it's not immediately clear that the Fortran support is usable alongside C++ in the same project. Ideally, the Fortran interface source files would be instrumented, too. I put some test documentation into profiler_mod, but it didn't seem to appear in the Doxygen output; I'm not sure that the Doxygen Fortran support is yet capable of recognising comments on interface blocks.