Allow overriding scale and offset values

[gerritholl]

Add an option to override the automatically calculated scale and offset tags.

• Partially closes ninjogeotiff writer should write gradient for P mode images satpy#2869
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)
• Fully documented (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.54%. Comparing base (8b71319) to head (f0d8d4c).
Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #177      +/-   ##
==========================================
+ Coverage   91.53%   91.54%   +0.01%     
==========================================
  Files          11       11              
  Lines        4062     4071       +9     
==========================================
+ Hits         3718     3727       +9     
  Misses        344      344              

Flag	Coverage Δ
unittests	91.54% <100.00%> (+0.01%)	⬆️

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[coveralls]

coverage: 91.796% (+0.02%) from 91.777%
when pulling f0d8d4c on gerritholl:feature-override-scale-offset
into c665c4d on pytroll:main.

[djhoese]

I think this is a pretty clean solution to the problem, but I was wondering if the simpler solution of turning them off would work for your NinJo-specific problem?

[mraspaud]

LGTM