This change structures the arguments of most functions, in particular
gray_main, into well-defined categories using derived types.
All types are defined in the gray_params.f90 (location subject to
change) and are organised as follows:
gray_parameters (statically allocated data)
├── antenna_parameters
├── ecrh_cd_parameters
├── equilibrium_parameters
├── misc_parameters
├── output_parameters
├── profiles_parameters
└── raytracing_parameters
gray_data - inputs of gray_main (dynamically-allocated arrays)
├── equilibrium_data
└── profiles_data
gray_results - outputs of gray_main (dynamically-allocated arrays)
