Fortran interface improvement

[olivhoenen]

A few things are bothering me in the Fortran interface as it is implemented at the moment:

• codebase layout: Fortran compatible functions are defined in src/cpp/muscle2 (fortranmuscle.h/.cpp), src/fortran contains only example programs

• binding: implementation of fortranmuscle.h/.cpp is outdated and lacks control over the compatible types, and requires to define C void functions with all parameters passed by address (non-standard C interface). Definition of wrapper functions using ISO_C_BINDING should be considered instead, for instance using the standard C interface (cmuscle.h) as target to our Fortran wrappers (in order to avoid code duplication and cherry-picking of API functions to be provided in a given interface)