Fix the slstr reader for consistency and tir view

[mraspaud]

This PR fixes the slstr reader for consistency and tir view

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff
• Fully documented

[djhoese]

So what does this fix allow that didn't work before? Was the problem that you could only load files from one view (o or n) per Scene?

[mraspaud]

The problem was for TIR channels that both n and o views where loaded at the same time and satpy tried to concatenate them, leading to trouble...

[djhoese]

Sounds good. Thanks for explaining.

[coveralls]

Coverage increased (+1.0%) to 73.967% when pulling fc53448 on fix-nc-slstr into 38fb064 on master.

[codecov]

Codecov Report

Merging #458 into master will increase coverage by 0.41%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   72.97%   73.38%   +0.41%     
==========================================
  Files         134      136       +2     
  Lines       17695    18910    +1215     
==========================================
+ Hits        12913    13878     +965     
- Misses       4782     5032     +250

Impacted Files	Coverage Δ
satpy/readers/nc_slstr.py	57.97% <0%> (-2.03%)	⬇️
satpy/tests/test_dataset.py	98.21% <0%> (-1.79%)	⬇️
satpy/readers/fci_fdhsi.py	18.7% <0%> (-1.3%)	⬇️
satpy/readers/li_l2.py	22.34% <0%> (-1.19%)	⬇️
satpy/readers/safe_sar_c.py	19.21% <0%> (-0.79%)	⬇️
satpy/tests/compositor_tests/__init__.py	98.01% <0%> (-0.48%)	⬇️
satpy/readers/native_msg.py	14.55% <0%> (-0.41%)	⬇️
satpy/tests/reader_tests/test_nc_nwcsaf.py	90.47% <0%> (ø)
satpy/tests/reader_tests/test_hrit_msg.py	94.66% <0%> (ø)
satpy/tests/test_yaml_reader.py	98.7% <0%> (+0.04%)	⬆️
... and 14 more

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