Resolve "Consolidate shared code"

This MR consolidates all code from mesonet and EEcosy that can be shared between models.

Those header files are moved to src/utils, alongside their tests. They are implemented in the Utopia::Utils namespace.

Furthermore, this MR addresses #2 (closed) by removing the GSL dependency and making FFTW optional.

Can this MR be accepted?

• Implementation ready
  • Files moved
  • Namespaces and includes adjusted
• Tests added or adjusted
  • Moved tests
  • Added test targets
  • Adjusted CI
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Code coverage should stay the same as before

• Ready for merging
  • Pipeline passes without warnings
  • All tests are carried out
  • cobertura and code coverage visualisation works
  • gcovr reporting the correct directories (excluding tests)
  • History cleaned-up or squash option set

Related issues

Closes #4 (closed), #2 (closed)