fix nan handling in color array

[knaaptime]

this fixes a bug where nan observations were being dropped silently (the conversion from string to numpy array was broken). This implements a fix and updates the tests for cases that include nans (and colors)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.6%. Comparing base (666accc) to head (3e4de03).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #229     +/-   ##
=======================================
+ Coverage   89.5%   89.6%   +0.1%     
=======================================
  Files          9       9             
  Lines       1127    1129      +2     
=======================================
+ Hits        1009    1012      +3     
+ Misses       118     117      -1     

Files with missing lines	Coverage Δ
mapclassify/util.py	92.6% <100.0%> (+4.6%)	⬆️

[martinfleis]

Can you write a test for this?