Add warning in 'awips_tiled' writer when 'units' are too long

[djhoese]

AWIPS has nice random limits to attributes depending on where and how it puts it in a database. Apparently the units attribute is limited to 26 characters. This PR adds a warning and forces the units to 26 characters.

CC @deeplycloudy @nbearson

[codecov]

Codecov Report

Merging #1802 (f473616) into main (9526d31) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1802      +/-   ##
==========================================
+ Coverage   92.91%   92.93%   +0.02%     
==========================================
  Files         265      265              
  Lines       39012    39112     +100     
==========================================
+ Hits        36247    36348     +101     
+ Misses       2765     2764       -1     

Flag	Coverage Δ
behaviourtests	4.75% <0.00%> (-0.02%)	⬇️
unittests	93.46% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_awips_tiled.py	98.48% <100.00%> (+0.06%)	⬆️
satpy/writers/awips_tiled.py	82.86% <100.00%> (+0.11%)	⬆️
satpy/tests/reader_tests/test_abi_l1b.py	99.25% <0.00%> (-0.75%)	⬇️
satpy/readers/netcdf_utils.py	100.00% <0.00%> (ø)
satpy/tests/reader_tests/test_hy2_scat_l2b_h5.py	100.00% <0.00%> (ø)
satpy/tests/reader_tests/test_netcdf_utils.py	94.89% <0.00%> (+0.10%)	⬆️
satpy/readers/hy2_scat_l2b_h5.py	98.21% <0.00%> (+0.21%)	⬆️
satpy/readers/abi_l1b.py	98.61% <0.00%> (+2.77%)	⬆️

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[coveralls]

Coverage increased (+0.003%) to 93.414% when pulling f473616 on djhoese:bugfix-awips-tiled-long-units into 2a81e65 on pytroll:main.