Set Doxygen documentation for C code.

[IshmaelBelghazi]

[tdhock]

documenting C code is technically not necessary for an R package, so I would advise to do this at the very end of GSOC only if you have extra time.

[IshmaelBelghazi]

Agreed. It is not essential at the moment. It may become more important later in the project when the C code gets more complex .