Add more descriptive names to AHI readers AreaDefinition names

[djhoese]

The previous names were things like "some_area_name". For users who want to put area names in their filenames ({area.area_id}) this makes the filename extremely ugly and not very useful. I took the information that I could find from the file and put it in the area definition. As far as everyone seems to know AHI HRIT files (himawaricast) these files only come in full disk (FLDK) variants so I hardcoded that name.

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[coveralls]

Coverage increased (+0.001%) to 74.067% when pulling d4d90b8 on djhoese:bugfix-ahi-area-name into ee6bc3a on pytroll:master.

[codecov]

Codecov Report

Merging #496 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #496      +/-   ##
==========================================
+ Coverage   74.08%   74.08%   +<.01%     
==========================================
  Files         137      137              
  Lines       18219    18220       +1     
==========================================
+ Hits        13498    13499       +1     
  Misses       4721     4721

Impacted Files	Coverage Δ
satpy/readers/ahi_hsd.py	45.6% <100%> (+0.3%)	⬆️
satpy/readers/hrit_jma.py	97.67% <100%> (ø)	⬆️

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