Add Python translation of NEON absorbance R package

[adrianateruelg]

This PR adds a complete Python translation of the NEON absorbance R package (absNEON).

What's included

All 8 functions have been translated:

• calcAbsRatio — absorbance ratio at two user-specified wavelengths
• calcE2E3 — E2:E3 ratio (250 nm ÷ 365 nm)
• calcSR — spectral slope ratio (275–295 nm ÷ 350–400 nm)
• calcSuva — SUVA at a user-specified wavelength
• calcSuva254 — SUVA at 254 nm with discrete absorbance fallback
• calcSuva280 — SUVA at 280 nm with discrete absorbance fallback
• calcSuva350 — SUVA at 350 nm
• formatWide — reformat absorbance data from long to wide format

Additional changes

• Shared helper functions (_helpers.py) for wavelength selection and Fe(III) correction
• Full test suite for all 8 functions
• README with usage examples and Fe correction methodology

Bug fix

The original R code contained a copy-paste error where the odd-wavelength check for wavelength2 incorrectly referenced wavelength1. This has been corrected in the Python translation.