feat: Reorganize Fortran interface

[LecrisUT]

A quick PR to re-organize the fortran api to be more legible and extendable. I am also considering splitting it into a header-source file, but at the moment there is little gain since CMake does not support bundling these files like with cxx-modules.

[lan496]

I would defer the Fortran interface on @atztogo .

[atztogo]

For me good. Thanks @LecrisUT.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (c168e89) 83.86% compared to head (bb3eb2d) 83.85%.

Files	Patch %	Lines
fortran/spglib_f08.F90	57.14%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #396      +/-   ##
===========================================
- Coverage    83.86%   83.85%   -0.02%     
===========================================
  Files           24       24              
  Lines         8180     8181       +1     
===========================================
  Hits          6860     6860              
- Misses        1320     1321       +1     

Flag	Coverage Δ
c_api	77.59% <ø> (ø)
fortran_api	56.14% <57.14%> (-0.01%)	⬇️
python_api	80.38% <ø> (ø)
unit_tests	1.24% <ø> (ø)

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