Add slstr nadir view definition

[TAlonglong]

Add SLSTR nadir view definition

• [ ] Tests added
• [ ] Tests passed
• [ ] Passes flake8 pyorbital
• [ ] Fully documented

It was harder than I thought to find the accurate scan angles of the SLSTR nadir view instrument. Anyone having a good pointer?

[codecov[bot]]

Codecov Report

Merging #81 (78ee740) into main (91794a6) will increase coverage by 1.58%. The diff coverage is 95.68%.

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   85.31%   86.89%   +1.58%     
==========================================
  Files          13       13              
  Lines        1893     2068     +175     
==========================================
+ Hits         1615     1797     +182     
+ Misses        278      271       -7     

Flag	Coverage Δ
unittests	86.89% <95.68%> (+1.58%)	:arrow_up:

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

Impacted Files	Coverage Δ
pyorbital/geoloc_instrument_definitions.py	72.61% <85.71%> (+1.28%)	:arrow_up:
pyorbital/tlefile.py	91.34% <90.82%> (+3.91%)	:arrow_up:
pyorbital/tests/test_orbital.py	96.28% <98.64%> (+0.82%)	:arrow_up:
pyorbital/orbital.py	86.97% <100.00%> (-0.19%)	:arrow_down:
pyorbital/tests/test_geoloc.py	94.64% <100.00%> (+0.30%)	:arrow_up:
pyorbital/tests/test_tlefile.py	97.99% <100.00%> (+0.55%)	:arrow_up:

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[TAlonglong]

Messed up merge and rebase. Need to fix this