Fix dtype promotion in channel inversion

[pnuu]

The XRImage.invert() promotes data to float64 when inversion parameters are given for each channel separately. The dtype was tested only for single inversion parameter case, not when they were given for each channel.

This PR splits the test case into two and adds the dtype check also for the per-channel case. And fixes the issue.

• Closes #xxxx (remove if there is no corresponding issue, which should only be the case for minor changes)
• 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 91.57%. Comparing base (f0d8d4c) to head (2ce875e).
Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #186      +/-   ##
==========================================
+ Coverage   91.54%   91.57%   +0.02%     
==========================================
  Files          11       11              
  Lines        4071     4081      +10     
==========================================
+ Hits         3727     3737      +10     
  Misses        344      344              

Flag	Coverage Δ
unittests	91.57% <100.00%> (+0.02%)	⬆️

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[coveralls]

coverage: 91.816% (+0.008%) from 91.808%
when pulling 2ce875e on pnuu:bugfix-invert-dtype-promotion
into c384f5b on pytroll:main.

[mraspaud]

Just a couple of questions, probably because I have not really understood everything

[pnuu]

Pushed two more commits to fix the issues in the comments.

[mraspaud]

LGTM!