Add v2a Fortran interface
This MR implements a Fortran interface for the v2a dynamic interface. There are two big changes to the C interface:
- in psetop/query_psetop output_sets must be preallocated (but not filled) by the user. n_output must be set to the number of strings in output_sets
- psetop_nb/query_psetop_nb cannot be implemented in Fortran as far as I can see, as there is explicit conversion of output_psets from C to Fortran needed