Add 'environment_prefix' to AWIPS tiled writer for flexible filenames

[djhoese]

Different processing environments may want different prefixes for files that will go to AWIPS. This is semi-standard for AWIPS tiled files (OR, OT, etc). This PR adds a new environment_prefix kwarg to the writer to control this for the GLM templates and fixes an issue where tile_number wasn't being provided to the user provided custom filename format string.

• Tests added
• Fully documented

CC @rayg-ssec @nbearson

[djhoese]

Closes #894

[codecov]

Codecov Report

Merging #1548 (461fc94) into master (b792f29) will increase coverage by 0.08%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master    #1548      +/-   ##
==========================================
+ Coverage   92.09%   92.17%   +0.08%     
==========================================
  Files         251      251              
  Lines       36711    36749      +38     
==========================================
+ Hits        33808    33874      +66     
+ Misses       2903     2875      -28     

Flag	Coverage Δ
behaviourtests	4.48% <0.00%> (-0.01%)	⬇️
unittests	92.71% <96.77%> (+0.08%)	⬆️

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Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_awips_tiled.py	98.24% <95.45%> (-0.25%)	⬇️
satpy/writers/awips_tiled.py	82.96% <100.00%> (+0.21%)	⬆️
satpy/readers/file_handlers.py	95.77% <0.00%> (-1.41%)	⬇️
satpy/tests/reader_tests/test_viirs_compact.py	100.00% <0.00%> (ø)
satpy/readers/viirs_compact.py	76.04% <0.00%> (+11.16%)	⬆️

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[djhoese]

Tests pass. Coverage went up. Codebeat is confused. Merging.