Update SLSTR reader to choose correct file for interpolated angles

[simonrp84]

At present, the SLSTR reader does not choose the correct file for interpolated angle datasets. It needs the cartesian coordinate file and chooses in by default, which is the IR (1km) resolution file.
This PR allows the reader to select the an file, which is at VIS resolution of 500m pixel size when required. This will enable angles to be correctly interpolated onto the SLSTR grid.

This partially addresses #1702 but does not allow angles to be interpolated correctly onto the IR grid as (I think) the 500m lat/lon grid is always selected.

• Tests added

[codecov]

Codecov Report

Merging #1706 (72e0ac4) into main (25993ff) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1706      +/-   ##
==========================================
+ Coverage   92.32%   92.55%   +0.22%     
==========================================
  Files         258      258              
  Lines       38125    38298     +173     
==========================================
+ Hits        35200    35445     +245     
+ Misses       2925     2853      -72     

Flag	Coverage Δ
behaviourtests	4.84% <0.00%> (-0.01%)	⬇️
unittests	93.09% <100.00%> (+0.22%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/readers/slstr_l1b.py	97.29% <100.00%> (+21.72%)	⬆️
satpy/tests/reader_tests/test_slstr_l1b.py	100.00% <100.00%> (ø)
satpy/tests/test_readers.py	98.86% <0.00%> (-0.37%)	⬇️
satpy/tests/reader_tests/test_nucaps.py	100.00% <0.00%> (ø)
satpy/tests/reader_tests/test_sar_c_safe.py	100.00% <0.00%> (ø)
satpy/readers/fci_l1c_fdhsi.py
satpy/tests/reader_tests/test_fci_l1c_fdhsi.py
satpy/readers/fci_l1c_nc.py	97.97% <0.00%> (ø)
satpy/tests/reader_tests/test_fci_l1c_nc.py	100.00% <0.00%> (ø)
satpy/tests/test_composites.py	99.88% <0.00%> (+<0.01%)	⬆️
... and 6 more

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[coveralls]

Coverage increased (+0.2%) to 93.039% when pulling 72e0ac4 on simonrp84:slstr into 25993ff on pytroll:main.

[mraspaud]

LGTM