reimplement rgbas

[knaaptime]

• fixes a bug when the 'scheme' argument is passed
• reimplement using matplotlib's vectorized method; much faster
• give back numpy instead of lists

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.5%. Comparing base (af62513) to head (3f53485).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #228     +/-   ##
=======================================
- Coverage   89.6%   89.5%   -0.1%     
=======================================
  Files          9       9             
  Lines       1125    1127      +2     
=======================================
+ Hits        1008    1009      +1     
- Misses       117     118      +1     

Files	Coverage Δ
mapclassify/util.py	88.0% <92.9%> (-3.3%)	⬇️

[knaaptime]

updated and renamed with an example if @martinfleis or @sjsrey want to have a look