Add low resolution file patterns for AHI HSD reader

[mherbertson]

This PR adds ability to read low resolution AHI HSD file format created by a sample JMA utility. As this utility requires the .DAT file, a bz2 format shouldn't be required.

https://www.data.jma.go.jp/mscweb/en/himawari89/space_segment/spsg_sample.html

[djhoese]

If this works for you then I am 👍 with it.

[codecov]

Codecov Report

Merging #1717 (648f76a) into main (1289aa3) will increase coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1717      +/-   ##
==========================================
+ Coverage   92.44%   92.55%   +0.10%     
==========================================
  Files         258      258              
  Lines       38279    38298      +19     
==========================================
+ Hits        35388    35445      +57     
+ Misses       2891     2853      -38     

Flag	Coverage Δ
behaviourtests	4.84% <ø> (-0.01%)	⬇️
unittests	93.09% <ø> (+0.10%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_slstr_l1b.py	100.00% <0.00%> (ø)
satpy/readers/slstr_l1b.py	97.29% <0.00%> (+21.72%)	⬆️

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[coveralls]

Coverage increased (+0.1%) to 93.039% when pulling 648f76a on mherbertson:feature-ahi-reader-add-extra-format into 1289aa3 on pytroll:main.

[mraspaud]

Don't forget to add you name to AUTHORS.md!

[djhoese]

This looks good to me. I'm going to wait for @mraspaud to decide if we merge this now and release it as part of 0.29.1 or wait for that release and include this as a "feature" in 0.30.0.