Fix some enhancements producing dask arrays wrapped in dask arrays

[djhoese]

I noticed while counting the number of dask computations that some enhancements were causing more computations than they used to. This was caused by some of the dask arrays producing dask arrays so the dask arrays were being passed to rasterio during geotiff saving which caused yet another compute.

The issue was that due to the refactoring in #2133, the functions were accidentally doing dask "stuff" (delayed functions or da.where) inside a function that was already map_blocks'd. This PR fixes all of that and adds checks to the tests to make sure it hopefully doesn't happen again in the future.

• Closes #xxxx
• Tests added
• Fully documented

[mraspaud]

LGTM

[codecov]

Codecov Report

Merging #2190 (ff5ff96) into main (7a23508) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2190   +/-   ##
=======================================
  Coverage   94.01%   94.01%           
=======================================
  Files         289      289           
  Lines       44613    44614    +1     
=======================================
+ Hits        41944    41945    +1     
  Misses       2669     2669           

Flag	Coverage Δ
behaviourtests	4.73% <0.00%> (-0.01%)	⬇️
unittests	94.67% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/enhancements/__init__.py	99.14% <100.00%> (-0.02%)	⬇️
satpy/tests/enhancement_tests/test_enhancements.py	99.64% <100.00%> (+<0.01%)	⬆️

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[coveralls]

Coverage increased (+0.0001%) to 94.624% when pulling ff5ff96 on djhoese:bugfix-btthreshold-enh into 7a23508 on pytroll:main.