Add pad_data functionality to FCI L1c FDHSI reader

[ameraner]

This PR plugs the fci_l1c_fdhsi reader into the GEOSegmentYAMLReader. With this, missing chunks (aka segments) are padded with nans using the scn.load([ds_name], pad_data=True) mechanism.

After switching to the GEOSegmentYAMLReader, the FCI tests were failing due to issues with the "hackyness" in the TestGEOSegmentYAMLReader. This was solved using the same unittest patch methods implemented in #1194.

• Tests added
• Tests passed
• Passes flake8 satpy

[codecov]

Codecov Report

Merging #1310 into master will increase coverage by 0.06%.
The diff coverage is 98.57%.

@@            Coverage Diff             @@
##           master    #1310      +/-   ##
==========================================
+ Coverage   89.98%   90.04%   +0.06%     
==========================================
  Files         219      220       +1     
  Lines       32105    32364     +259     
==========================================
+ Hits        28889    29143     +254     
- Misses       3216     3221       +5     

Impacted Files	Coverage Δ
satpy/readers/yaml_reader.py	95.23% <95.00%> (-0.07%)	⬇️
satpy/tests/reader_tests/test_fci_l1c_fdhsi.py	100.00% <100.00%> (ø)
satpy/tests/test_yaml_reader.py	99.83% <100.00%> (+0.02%)	⬆️
satpy/tests/reader_tests/test_ami_l1b.py	96.29% <0.00%> (-0.90%)	⬇️
satpy/node.py	94.48% <0.00%> (-0.36%)	⬇️
satpy/tests/test_scene.py	99.72% <0.00%> (-0.01%)	⬇️
satpy/tests/test_dataset.py	100.00% <0.00%> (ø)
satpy/tests/reader_tests/test_utils.py	100.00% <0.00%> (ø)
satpy/tests/reader_tests/test_ahi_hsd.py	100.00% <0.00%> (ø)
satpy/tests/reader_tests/test_iasi_l2_so2_bufr.py	97.18% <0.00%> (ø)
... and 8 more

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[coveralls]

Coverage increased (+0.03%) to 90.048% when pulling 118fd36 on ameraner:feature-padding_support_fci_reader into 4924cba on pytroll:master.

[mraspaud]

LGTM

[ameraner]

Thanks a lot for the reviews and the merge!