add a function to calculate cell size

[tiemvanderdeure]

Closes #415

[rafaqz]

Thanks!! So excited to have this method in Rasters, thanks for writing it up.

I made a few comments on style and naming, and some doc string changes (that you can just merge in in the interface if you want them)

The tests are failing because we forgot to pass through the function name in the extensions error method - we should pass in e.g. cellarea as f and use it in the MethodError

[tiemvanderdeure]

Thanks for your comments! The MethodError should work now.

[codecov-commenter]

Codecov Report

Merging #508 (0f41810) into main (bde22ed) will decrease coverage by 0.83%.
Report is 1 commits behind head on main.
The diff coverage is 12.50%.

@@            Coverage Diff             @@
##             main     #508      +/-   ##
==========================================
- Coverage   81.25%   80.43%   -0.83%     
==========================================
  Files          49       50       +1     
  Lines        4135     4170      +35     
==========================================
- Hits         3360     3354       -6     
- Misses        775      816      +41     

Files Changed	Coverage Δ
ext/RastersArchGDALExt/RastersArchGDALExt.jl	100.00% <ø> (ø)
ext/RastersArchGDALExt/cellsize.jl	0.00% <0.00%> (ø)
src/Rasters.jl	100.00% <ø> (ø)
src/extensions.jl	70.00% <75.00%> (-14.62%)	⬇️

... and 1 file with indirect coverage changes

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[rafaqz]

Thanks!

FYI usually a PR is merged when all the comments are resolved one way or another, by clicking the "resolve conversation" button if you think its been addressed :)

[asinghvi17]

Just to confirm, this is the Haversine distance, i.e., area by great circle calculation, not area by true geodesic (a la Karney/Vincenty)?

[tiemvanderdeure]

It uses Girard's theorem. So it doesn't use Haversine distance exactly, but it is an approximation for spheres.