Clarify safety of calling Fortran libraries without BIND(C)

[krystophny]

One should make sure that the way arguments are passed by reference are always consistent. At least for gfortran this seems to be the case. See

https://gcc.gnu.org/onlinedocs/gfortran/Argument-passing-conventions.html
https://gcc.gnu.org/onlinedocs/gfortran/Code-Gen-Options.html
https://thinkingeek.com/2017/01/14/gfortran-array-descriptor/

Subroutines eat pointers with references to data. Array types are specified uniquely via structs. Function return values of primitive types are by value, and as a pointer in more complicated types. Nowadays (post-g77 and f2c) C99 COMPLEX datatype is used.