Change default type of AWIPS tiled writer to int16 + _Unsigned

[djhoese]

Apparently AWIPS can't handle signed 16-bit integers.

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2151 (42e4cd3) into main (ada50bb) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2151      +/-   ##
==========================================
+ Coverage   93.91%   93.95%   +0.03%     
==========================================
  Files         283      285       +2     
  Lines       43258    43755     +497     
==========================================
+ Hits        40626    41109     +483     
- Misses       2632     2646      +14     

Flag	Coverage Δ
behaviourtests	4.73% <0.00%> (-0.05%)	⬇️
unittests	94.62% <100.00%> (+0.06%)	⬆️

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Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_awips_tiled.py	99.05% <100.00%> (+<0.01%)	⬆️
satpy/writers/awips_tiled.py	84.08% <100.00%> (+0.03%)	⬆️
satpy/utils.py	25.00% <0.00%> (-0.40%)	⬇️
satpy/tests/test_utils.py	100.00% <0.00%> (ø)
satpy/readers/gpm_imerg.py	100.00% <0.00%> (ø)
satpy/tests/modifier_tests/test_parallax.py	99.71% <0.00%> (ø)
satpy/modifiers/parallax.py	99.15% <0.00%> (ø)
satpy/composites/__init__.py	90.27% <0.00%> (+0.13%)	⬆️
satpy/tests/utils.py	94.38% <0.00%> (+0.56%)	⬆️

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[coveralls]

Coverage increased (+0.0006%) to 94.571% when pulling 42e4cd3 on djhoese:bugfix-awips-tiled-physelem into 8e039d6 on pytroll:main.

[mraspaud]

LGTM, thanks for cleaning up old comments.