Sar-c reader optimization

[mraspaud]

This PR optimizes the Sar-C reader, among other things by using rioxarray for reading the data.

• Tests added
• Fully documented

A simple benchmarking on my laptop reports a reduction of 33% of the memory usage with this PR. The reading is also about 10% faster.

[codecov]

Codecov Report

Merging #1580 (85f10cf) into master (5504ea4) will increase coverage by 0.25%.
The diff coverage is 96.01%.

@@            Coverage Diff             @@
##           master    #1580      +/-   ##
==========================================
+ Coverage   92.54%   92.79%   +0.25%     
==========================================
  Files         251      251              
  Lines       36801    37137     +336     
==========================================
+ Hits        34058    34463     +405     
+ Misses       2743     2674      -69     

Flag	Coverage Δ
behaviourtests	4.45% <0.00%> (-0.05%)	⬇️
unittests	92.93% <96.01%> (+0.25%)	⬆️

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Impacted Files	Coverage Δ
satpy/readers/sar_c_safe.py	86.88% <88.58%> (+34.11%)	⬆️
satpy/readers/utils.py	90.27% <93.10%> (+0.09%)	⬆️
satpy/readers/nwcsaf_nc.py	61.27% <94.73%> (+0.86%)	⬆️
satpy/readers/satpy_cf_nc.py	97.56% <95.23%> (-1.04%)	⬇️
satpy/readers/ahi_hsd.py	97.29% <100.00%> (+0.43%)	⬆️
satpy/readers/yaml_reader.py	95.87% <100.00%> (+0.52%)	⬆️
satpy/scene.py	91.46% <100.00%> (-0.19%)	⬇️
satpy/tests/reader_tests/test_ahi_hsd.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_sar_c_safe.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_satpy_cf_nc.py	100.00% <100.00%> (ø)
... and 8 more

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[djhoese]

LGTM after skimming