feat: add support for single-color colormaps with constant mapping behavior

[tlambert03]

closes #139

hey @aafkegros, here's one possible fix to #139. Let me know how this would work for you. And again, still happy to iterate on this

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.56%. Comparing base (954b1ca) to head (eedc86c).

Files with missing lines	Patch %	Lines
src/cmap/_colormap.py	83.33%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (83.33%) is below the target coverage (85.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #140      +/-   ##
==========================================
- Coverage   95.56%   93.56%   -2.00%     
==========================================
  Files         168      168              
  Lines        2186     2191       +5     
==========================================
- Hits         2089     2050      -39     
- Misses         97      141      +44     

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[aafkegros]

I think this is a valid solution to solve that the single-valued colormap did not return a constant-valued map. However, l do think it's introducing more exceptions to the Colormap model than allowing single-valued colormaps would.

Given that the interpolation at calling behaviour would also work for a single valued color, and that would avoid inserting a new value and throwing away the color stop.

I can open a PR for my envisioned solution, but it will take a bit of time as I'm a bit busy in other tasks 😅

[tlambert03]

Sure, I'd be happy to see it. No hurry (as long as you're not in a hurry for a fix! 😃)