Fix bug in OLCI reader that caused multiple error messages to print

[simonrp84]

In issue #944 I describe how the OLCI reader prints numerous error messages to the screen during processing. This PR fixed the error messages by switching to the default xarray engine for netCDF4. I have also added functions that explicitly close the netcdf files at the end of the processing. This may or may not be needed (I'm not sure) but I thought I'd copy what the GOES-ABI reader does here.

Lastly, I also split one line of code onto two, which was previously giving a flake8 style error for being too long.

• Closes Multiple errors when processing OLCI data. #944
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage decreased (-0.02%) to 85.386% when pulling cf1ab72 on simonrp84:master into 325848a on pytroll:master.

[coveralls]

Coverage increased (+0.4%) to 85.84% when pulling af639bb on simonrp84:master into 325848a on pytroll:master.

[codecov]

Codecov Report

Merging #945 into master will increase coverage by 0.42%.
The diff coverage is 90.24%.

@@            Coverage Diff             @@
##           master     #945      +/-   ##
==========================================
+ Coverage   85.41%   85.84%   +0.42%     
==========================================
  Files         172      172              
  Lines       26048    26222     +174     
==========================================
+ Hits        22248    22509     +261     
+ Misses       3800     3713      -87

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_olci_nc.py	95.6% <100%> (+1.31%)	⬆️
satpy/readers/olci_nc.py	90.9% <76.47%> (+42.57%)	⬆️
satpy/readers/avhrr_l1b_gaclac.py	90.14% <0%> (-1.41%)	⬇️
satpy/tests/test_multiscene.py	97.84% <0%> (ø)	⬆️
satpy/multiscene.py	89.13% <0%> (ø)	⬆️
satpy/readers/modis_l2.py	98.52% <0%> (ø)	⬆️
satpy/readers/seviri_l1b_hrit.py	92.3% <0%> (ø)	⬆️
satpy/tests/enhancement_tests/test_enhancements.py	98.72% <0%> (+1.81%)	⬆️
satpy/enhancements/__init__.py	97.05% <0%> (+7.65%)	⬆️

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[mraspaud]

Should we have the default engine to None so as to use xarray's default, or do we want to hardcode it to netcdf4 ?

[simonrp84]

Should we have the default engine to None so as to use xarray's default, or do we want to hardcode it to netcdf4 ?

Good idea, I've changed it to None. I have also added/modified the tests so that coverage is improved a bit.

[mraspaud]

LGTM, thanks for cleaning up the deprecated stuff :) I just have a few cosmetic items to point out.