ENH: add POC of a cover marker

[theroggy]

First, very rough version...

[codecov]

Codecov Report

Attention: Patch coverage is 78.14570% with 33 lines in your changes missing coverage. Please review.

Project coverage is 64.07%. Comparing base (b34fd06) to head (6612d25).

Files with missing lines	Patch %	Lines
cropclassification/calc_cover.py	76.00%	30 Missing ⚠️
cropclassification/preprocess/timeseries.py	77.77%	2 Missing ⚠️
cropclassification/util/mosaic_util.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
+ Coverage   63.77%   64.07%   +0.30%     
==========================================
  Files          36       37       +1     
  Lines        4662     4791     +129     
==========================================
+ Hits         2973     3070      +97     
- Misses       1689     1721      +32     

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