Add altitude in the list of dataset for OLCI.nc

[ghislainp]

it add support for OLCI to read altitude

[codecov]

Codecov Report

Merging #1555 (d005e05) into master (e0f1fb8) will increase coverage by 0.52%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master    #1555      +/-   ##
==========================================
+ Coverage   91.65%   92.17%   +0.52%     
==========================================
  Files         248      251       +3     
  Lines       36172    36764     +592     
==========================================
+ Hits        33155    33889     +734     
+ Misses       3017     2875     -142     

Flag	Coverage Δ
behaviourtests	4.49% <9.65%> (+0.13%)	⬆️
unittests	92.71% <96.55%> (+0.51%)	⬆️

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Impacted Files	Coverage Δ
satpy/dataset/data_dict.py	90.09% <ø> (ø)
satpy/readers/ahi_hsd.py	96.86% <ø> (ø)
satpy/readers/ahi_l1b_gridded_bin.py	99.05% <ø> (ø)
satpy/readers/ami_l1b.py	96.99% <ø> (ø)
satpy/readers/fci_l2_nc.py	92.80% <ø> (ø)
satpy/readers/goes_imager_nc.py	65.46% <0.00%> (ø)
satpy/readers/iasi_l2_so2_bufr.py	94.62% <ø> (ø)
satpy/readers/mimic_TPW2_nc.py	80.95% <ø> (ø)
satpy/readers/nwcsaf_nc.py	60.40% <0.00%> (ø)
satpy/readers/olci_nc.py	91.42% <0.00%> (ø)
... and 62 more

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[mraspaud]

LGTM