Fix 'abi_l1b' reader keeping _Unsigned attribute

[djhoese]

The 'abi_l1b' reader currently keeps the _Unsigned attribute that can be present in ABI L1B files. This attribute has special meaning in NetCDF files and should not be included in the metadata shown to the user. This can/could also be an issue when saving to NetCDF files with the cf writer.

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[codecov]

Codecov Report

Merging #838 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #838      +/-   ##
==========================================
+ Coverage   83.96%   83.97%   +<.01%     
==========================================
  Files         167      167              
  Lines       24566    24567       +1     
==========================================
+ Hits        20628    20630       +2     
+ Misses       3938     3937       -1

Impacted Files	Coverage Δ
satpy/readers/abi_l1b.py	94.48% <100%> (+0.04%)	⬆️
satpy/scene.py	90.65% <0%> (+0.17%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 28b2628...6b80d80. Read the comment docs.

[coveralls]

Coverage increased (+0.02%) to 83.974% when pulling 6b80d80 on djhoese:bugfix-abi-unsigned into 28b2628 on pytroll:master.

[mraspaud]

LGTM