Fix dtype preservation in astronomy functions

[djhoese]

Redo of #155 without type annotation

• Closes #xxxx
• Tests added
• Tests passed
• Passes flake8 pyorbital
• Fully documented

[codecov]

Codecov Report

Attention: Patch coverage is 90.66667% with 7 lines in your changes missing coverage. Please review.

Project coverage is 87.99%. Comparing base (18340ca) to head (3247b15).
Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
pyorbital/astronomy.py	88.23%	4 Missing ⚠️
pyorbital/tests/test_astronomy.py	92.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
+ Coverage   87.51%   87.99%   +0.48%     
==========================================
  Files          14       14              
  Lines        2210     2241      +31     
==========================================
+ Hits         1934     1972      +38     
+ Misses        276      269       -7     

Flag	Coverage Δ
unittests	87.99% <90.66%> (+0.48%)	⬆️

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[coveralls]

coverage: 87.933% (+0.4%) from 87.562%
when pulling 3247b15 on djhoese:bugfix-dtype-promotion2
into 83e029a on pytroll:main.

[djhoese]

As discussed on slack, I've got the OK to merge this since it just includes the "good" parts of the other PR.