This converts the last remaining warnings to use the logging system.
Also drops `catand` and replace it with the intrinsic `atan`, which
supports complex as well as real numbers.
Note: before 3eab989d the `catand` function was actually incorrent!
The definition of arctan(z) can be obtained starting from the identity
d/dz arctan(z) = 1/(1 + z²) = ½ [1/(1 + iz) + 1/(1 - iz)],
integrating and using the definition log(z) = ∫₁^z dz/z,
arctan(z) = -i/2 [log(1 + iz) - log(1 - iz)].
If log is the principal branch, log(z) = log|z| + i arg(z), then
arctan(z) = -i/2 log(w) = 1/2 arg(w) -i/2 log|w|
where w = (1 + iz)/(1 - iz). Finally, the real part is
Re arctan(z) = 1/2 atan2(2Re(z), 1 - |z|²).
The term -|z|² is missing from the `catand` definition of GRAY,
but is present in the original Fortran 77 code from [SLATEC]:
it has probably been lost in the translation.
[SLATEC]: https://people.math.sc.edu/Burkardt/f_src/slatec/slatec.f90
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)
