Avoid repeated extinction law construction in redden!/deredden!

[Akshat1000Sharma]

This PR improves the implementation of redden! and deredden! in src/transforms/redden.jl.

Previously the implementation broadcasted directly over the extinction law type:

@. f = DustExtinction.redden(law, s, f; Rv, Av)

When law is provided as a Type (e.g. the default DustExtinction.CCM89), the broadcast dispatch calls the type-based method of DustExtinction.redden for every wavelength element. This leads to constructing the extinction law instance (e.g. CCM89(Rv=3.1)) repeatedly.

This PR instead constructs the extinction law instance once and then applies the broadcast using that instance:

law_instance = law isa DustExtinction.ExtinctionLaw ? law : law(Rv=Rv)
@. f = DustExtinction.redden(law_instance, s, f; Av)

The same change is applied to deredden!.

Benefits

• Avoids repeated construction of extinction law objects
• Keeps the public API unchanged
• Continues delegating reddening calculations to DustExtinction.jl

Future work

There is ongoing upstream work adding in-place versions of these functions in DustExtinction.jl:

JuliaAstro/DustExtinction.jl#84

Once those functions (redden! / deredden!) are available in a released version, Spectra.jl could simplify this further by delegating directly to those implementations. (Once that happens, it will also fix #45 )

Testing

All existing tests (224/224) pass successfully.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.85%. Comparing base (58828d3) to head (db9daea).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
+ Coverage   90.74%   90.85%   +0.11%     
==========================================
  Files           8        8              
  Lines         162      164       +2     
==========================================
+ Hits          147      149       +2     
  Misses         15       15              

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