Bugfix (FY4A AGRI): Correct the type of orbital_parameters to float

[zxdawn]

• Closes Wrong data type of orbital_parameters in FY4A AGRI reader #1243
• Tests added
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.001%) to 90.028% when pulling 0e7a56a on zxdawn:fy4a into 9261a1c on pytroll:master.

[coveralls]

Coverage increased (+0.002%) to 90.028% when pulling 2f130f2 on zxdawn:fy4a into 9261a1c on pytroll:master.

[codecov]

Codecov Report

Merging #1244 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1244   +/-   ##
=======================================
  Coverage   90.02%   90.02%           
=======================================
  Files         218      218           
  Lines       31263    31269    +6     
=======================================
+ Hits        28145    28151    +6     
  Misses       3118     3118           

Impacted Files	Coverage Δ
satpy/readers/agri_l1.py	99.18% <ø> (ø)
satpy/tests/reader_tests/test_agri_l1.py	100.00% <100.00%> (ø)

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[sfinkens]

Nice work, thank you! Just one little wish.

[sfinkens]

Looks good to me now!

[mraspaud]

LGTM