FCI: fix areadef when only pixel quality asked

[gerritholl]

When reading only pixel quality fields from FCI, make sure it can still
correctly calculate the area definition. Closes #1229.

• Closes FCI reader can read pixel_quality flags only after reading corresponding channel data #1229
• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented

[coveralls]

Coverage increased (+0.004%) to 89.957% when pulling d690abe on gerritholl:fci-read-quality-first into e10908a on pytroll:master.

[codecov]

Codecov Report

Merging #1230 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1230   +/-   ##
=======================================
  Coverage   89.95%   89.95%           
=======================================
  Files         208      208           
  Lines       30587    30598   +11     
=======================================
+ Hits        27514    27525   +11     
  Misses       3073     3073           

Impacted Files	Coverage Δ
satpy/readers/fci_l1c_fdhsi.py	98.10% <100.00%> (+0.03%)	⬆️
satpy/tests/reader_tests/test_fci_l1c_fdhsi.py	100.00% <100.00%> (ø)

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[gerritholl]

Apparently, this also avoids the segmentation fault reported in pytroll/pyresample#274

[mraspaud]

LGTM