APIs for external libraries

[fedebenelli]

This pull request focuses on the wrapping of external libraries, at the end of the pull requests it should include

• [ ] Wrappers to known libraries:

  • [ ] FeOs

    • [ ] Object saving pointer
    • [x] json string generator
    • [ ] Model calling interface

  • [ ] teqp

    • [ ] Object saving pointer
    • [ ] json string generator
    • [ ] Model calling interface

• [ ] Maybe some C API standard type?

• [ ] Move the C-API of yaeos-python to be called from other C libraries

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 42.69663% with 51 lines in your changes missing coverage. Please review.

Project coverage is 91.97%. Comparing base (e8aa28b) to head (5765ab1).

Files	Patch %	Lines
src/fitting/fit_nrtl_mhv.f90	0.00%	38 Missing :warning:
src/models/external_apis/feos.f90	75.75%	8 Missing :warning:
src/fitting/fitting.f90	58.33%	4 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #87 +/- ## ========================================== - Coverage 93.60% 91.97% -1.63% ========================================== Files 53 56 +3 Lines 2392 2468 +76 Branches 179 179 ========================================== + Hits 2239 2270 +31 - Misses 105 150 +45 Partials 48 48 ```

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