Fix MODIS readers applying add_offset incorrectly

[djhoese]

It was pointed out on the mailing list that when reading MODIS L2 (MOD06) cloud top temperature that the resulting values were nonsensical. This is due to us applying scale_factor/add_offset in the wrong way. We were doing:

data = byte_value * scale_factor + add_offset

but we should be doing:

data = (byte_value - add_offset) * scale_factor

because MODIS.

This PR fixes this issue and others. While adding checks to the tests to verify that what was happening was actually happening, I noticed that byte mask conversions were actually converting dask to numpy arrays. I've fixed this issue as well by wrapping these operations in a da.map_blocks call.

Note: This PR does not include the test fixes in #2141 so the tests will fail until that is merged.

• Closes #xxxx
• Tests added
• Fully documented

[mraspaud]

Looks goods, just two minor suggestions.

[simonrp84]

Just tested this on MODIS L1 data, no effect on the results - which is what we expect as these use a 0 add_offset.

[codecov]

Codecov Report

Merging #2142 (442cccd) into main (607c25b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2142   +/-   ##
=======================================
  Coverage   93.88%   93.88%           
=======================================
  Files         283      283           
  Lines       43106    43123   +17     
=======================================
+ Hits        40470    40488   +18     
+ Misses       2636     2635    -1     

Flag	Coverage Δ
behaviourtests	4.78% <0.00%> (-0.01%)	⬇️
unittests	94.54% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
satpy/readers/hdfeos_base.py	92.27% <100.00%> (+0.40%)	⬆️
satpy/readers/modis_l2.py	89.81% <100.00%> (+0.29%)	⬆️
satpy/tests/reader_tests/_modis_fixtures.py	97.61% <100.00%> (+0.01%)	⬆️
satpy/tests/reader_tests/test_modis_l1b.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_modis_l2.py	100.00% <100.00%> (ø)

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[coveralls]

Coverage increased (+0.005%) to 94.484% when pulling 442cccd on djhoese:bugfix-modis-scale-offset into 607c25b on pytroll:main.