skip missingval when calling extract with skipmissing = true

[tiemvanderdeure]

Currently only values equal to missing are skipped. This PR makes it so that values equal to the missingval of the input raster are skipped as well.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.37%. Comparing base (7ab31df) to head (0aa3b60).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/methods/extract.jl	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #617      +/-   ##
==========================================
+ Coverage   81.34%   81.37%   +0.02%     
==========================================
  Files          61       61              
  Lines        4337     4343       +6     
==========================================
+ Hits         3528     3534       +6     
  Misses        809      809              

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