Make get_observer_look work on oblate earth

[mraspaud]

The pyorbital code to compute the observer look was making the assumption that the earth was spherical. While the differences are quite small, we implement here the oblateness of the earth into the function.

• Closes Changed get_observer_look function to match Oribtal.get_observer_look #44
• Tests added
• Tests passed
• Passes flake8 pyorbital
• Fully documented

[codecov]

Codecov Report

Merging #87 (6785eed) into main (e215868) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
- Coverage   85.67%   85.55%   -0.12%     
==========================================
  Files          13       13              
  Lines        1947     1932      -15     
==========================================
- Hits         1668     1653      -15     
  Misses        279      279              

Flag	Coverage Δ
unittests	85.55% <100.00%> (-0.12%)	⬇️

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Impacted Files	Coverage Δ
pyorbital/orbital.py	86.60% <100.00%> (-0.37%)	⬇️

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