Allow linear alpha stretching in certain conditions

[mraspaud]

This PR allows alpha channel to be linearly stretched if explicitly passed cut-off values.

• Closes Versions above 1.21.0 will not produce IR Images #162
• Tests added
• Tests passed
• 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 ✅

Comparison is base (5e6bd79) 91.57% compared to head (97231c0) 91.63%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
+ Coverage   91.57%   91.63%   +0.06%     
==========================================
  Files          12       12              
  Lines        4014     4043      +29     
==========================================
+ Hits         3676     3705      +29     
  Misses        338      338              

Flag	Coverage Δ
unittests	91.63% <100.00%> (+0.06%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[coveralls]

coverage: 91.867% (+0.06%) from 91.808%
when pulling 97231c0 on mraspaud:fix-alpha-stretching
into 36d98aa on pytroll:main.

[mraspaud]

Confirmed to fix the problem in #162 by OP

[djhoese]

LGTM