Do not apply linear stretch to alpha band

[mraspaud]

This PR fixes trollimage stretching the alpha band along with the other bands when using linear.

• 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)

[codecov]

Codecov Report

Merging #145 (47a0f2e) into main (f211ef7) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   91.18%   91.23%   +0.04%     
==========================================
  Files          11       11              
  Lines        3847     3867      +20     
==========================================
+ Hits         3508     3528      +20     
  Misses        339      339              

Flag	Coverage Δ
unittests	91.23% <100.00%> (+0.04%)	⬆️

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

Files	Coverage Δ
trollimage/tests/test_image.py	100.00% <100.00%> (ø)
trollimage/xrimage.py	89.43% <100.00%> (+0.19%)	⬆️

[coveralls]

coverage: 91.479% (+0.04%) from 91.435% when pulling 47a0f2e on mraspaud:fix-alpha into f211ef7 on pytroll:main.

[djhoese]

Looks good. My only comment on this is what if I want to linear scale the alpha channel? Don't let this question stop this from being merged...but still.

[mraspaud]

Looks good. My only comment on this is what if I want to linear scale the alpha channel? Don't let this question stop this from being merged...but still.

I can't really imagine why someone would want to stretch the alpha channel, but yeah, this PR removes that possibility

[mraspaud]

I'll just merge this, and will fix this if the need to stretch alpha arises.