Anticipate filehandler sorting in GEOSegmentYAMLReader to have sorted handlers also with pad_data=False

[ameraner]

This PR fixes the issue reported in #2588 by making the segment sorting happen early during create_filehandlers, so that it acts for both pad_data=True and False.

The root cause was that the filehandler sorting by segment was done only in the default pad_data=True branch inside the padding mechanism of GEOSegmentYAMLReader. When calling e.g. scn.load(['ir_105'], pad_data=False, upper_right_corner='NE'), in the previous implementation the filehandlers were being sorted (in the base FileYAMLReader.create_filehandlers) only by start_time (which is always the same after #2420), and then alphabetically by filenames. This is problematic for FCI where sorting alphabetically means sorting by processing_time, and there are cases where the processing_time order does not correspond to the chunk order (due to parallel processing), causing the issue.

Note that even without #2420, some FCI segments have the same filename start_time, hence the bug would have happened anyway.

• Closes FCI chunks/segments out of order if pad_data=False #2588
• Tests added
• Fully documented

[ameraner]

This snippet of code

scn = Scene(filenames=filenames, reader='fci_l1c_nc')
scn.load(['ir_105'], pad_data=False, upper_right_corner='NE')
scn.save_dataset('ir_105', filename='ir105_nopad.png')

produces this before this PR

and this after this PR

[pnuu]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c5c18cf) 95.32% compared to head (36aa471) 95.35%.
Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2692      +/-   ##
==========================================
+ Coverage   95.32%   95.35%   +0.03%     
==========================================
  Files         371      371              
  Lines       52441    52485      +44     
==========================================
+ Hits        49988    50049      +61     
+ Misses       2453     2436      -17     

Flag	Coverage Δ
behaviourtests	4.15% <3.70%> (-0.01%)	⬇️
unittests	95.97% <100.00%> (+0.03%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 7221665410

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• 27 of 27 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on bugfix_add_filehandler_sorting_with_paddata_false at 95.922%

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Totals
Change from base Build 7220123684:	95.9%
Covered Lines:	50175
Relevant Lines:	52308

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💛 - Coveralls

[mraspaud]

LGTM