Fix handling of nans in lookup table enhancement

[avalentino]

The incorrect handling of nans in the lookup table enhancement function causes warning messages in almost all architectures while an error is raised on riscv64.

Patch kindly provided by Aurelien Jarno in
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1121879

• Closes No explicit handling of nans in lookup table enhancement #2934
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.33%. Comparing base (03a04e8) to head (7b86d9c).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3315      +/-   ##
==========================================
- Coverage   96.33%   96.33%   -0.01%     
==========================================
  Files         463      463              
  Lines       58901    58880      -21     
==========================================
- Hits        56742    56721      -21     
  Misses       2159     2159              

Flag	Coverage Δ
behaviourtests	3.60% <0.00%> (+<0.01%)	⬆️
unittests	96.42% <100.00%> (-0.01%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 20133190717

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 96.409%

---

Files with Coverage Reduction	New Missed Lines	%
enhancements/colormap.py	1	98.72%

Totals
Change from base Build 20069710983:	0.0%
Covered Lines:	56599
Relevant Lines:	58707

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💛 - Coveralls