Abstracting optimizer interface

[fedebenelli]

Abstracting the optimization API to ease the inclusion of other optimizers besides nlopt

TODO

• [x] Provide a simpler default optimizer to reduce dependencies (Maybe use MINPACK? https://github.com/fortran-lang/minpack)
• [x] Keep NLOPT wrapper just as an example
• [x] Update tests

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.95652% with 6 lines in your changes missing coverage. Please review.

Project coverage is 93.25%. Comparing base (e0469ab) to head (84913ee). Report is 2 commits behind head on dev.

Files	Patch %	Lines
src/fitting/fitting.f90	66.66%	3 Missing and 1 partial :warning:
src/auxiliar/optimizers.f90	87.50%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #106 +/- ## ========================================== - Coverage 93.41% 93.25% -0.16% ========================================== Files 57 58 +1 Lines 2687 2683 -4 Branches 194 193 -1 ========================================== - Hits 2510 2502 -8 - Misses 127 131 +4 Partials 50 50 ```

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[fedebenelli]

Added PowellOptim dependency that uses Powell Optimizers (Like COBYLA) and removed nlopt, keeping it as an example of wrapping