Remove Invalid Metadata From ACSPO Reader

[wroberts4]

_FillValue is not used since all invalid data is mapped to nan. _FillValue, valid_max , and valid_min
cause misinformation to other software because of the former statement.

• Closes #xxxx
• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

Merging #843 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #843      +/-   ##
==========================================
- Coverage   84.09%   84.09%   -0.01%     
==========================================
  Files         167      167              
  Lines       24546    24549       +3     
==========================================
+ Hits        20642    20644       +2     
- Misses       3904     3905       +1

Impacted Files	Coverage Δ
satpy/readers/acspo.py	86.25% <100%> (+0.53%)	⬆️
satpy/scene.py	90.47% <0%> (-0.18%)	⬇️

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[djhoese]

Thanks!

[coveralls]

Coverage increased (+0.02%) to 84.093% when pulling f5be913 on wroberts4:master into 7360647 on pytroll:master.