Progressively add Doxygen doc strings to adflow's fortran routines

[joanibal]

Description

As discussed in the maintenance meeting, adding doc strings to the Fortran subroutines would greatly improve the readability of the low-level code.

The comments will use Doxygen formatting so that we can compile them into additional documentation for Adflow.

The initial though was to do a progressive roll out since the code base is so large.
To accomplish this the maintainers are responsible for making sure that when someone edits an existing Fortran function in Adflow, they also add a doc string.

[ewu63]

How good is the Fortran support in Doxygen? There is also FORD which is being used by several modern Fortran projects, but I hesitate to recommend it since I don't think it provides other features from Doxygen such as the call graphs.