Embed Oudin's Fortran code

The PREMHYCE project need a faster version of the PE_Oudin() function.

The vectorization of the code is not satisfactory, it would be preferable to integrate the Fortran version of the code. I think that is a good idea to keep the R version of the code to facilitate the comprehension by the users. In this case we can add an argument to switch from the R code to the Fortran code.