Support for changing the Voigt implementation

[ConorMacBride]

When initialising the IBIS8542Model, a impl kwarg can now be passed to specify the function used to evaluate the Voigt function. The Faddeeva Voigt function is also added, using the SciPy implementation, however, the default implementation remains the integration method such that the code is fully backwards compatible.

Also, additional kwargs passed to the fit method are now passed through to SciPy curve_fit, which allows for the least squares configuration to be changed with ease.

[codecov]

Codecov Report

Merging #51 (1641217) into main (810d864) will decrease coverage by 0.05%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
- Coverage   95.07%   95.02%   -0.06%     
==========================================
  Files          18       18              
  Lines        1382     1367      -15     
==========================================
- Hits         1314     1299      -15     
  Misses         68       68              

Impacted Files	Coverage Δ
src/mcalf/models/base.py	84.87% <50.00%> (ø)
src/mcalf/models/ibis.py	92.12% <100.00%> (+0.14%)	⬆️
src/mcalf/profiles/voigt.py	96.10% <100.00%> (-0.74%)	⬇️
src/mcalf/visualisation/spec.py	100.00% <100.00%> (ø)

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