Various optimizations to colorspace code

[djhoese]

I did a few little ones to allow a clearer path for the compiler to recognized clipping a value in one of the conversions to 0.0 and 1.0. The bigger update I did to have everything use a single memory view as input and output hopefully saves memory...maybe, but it doesn't seem to improve execution time which is surprising and extremely disappointing.

• 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

Merging #143 (7d256a4) into main (06e2ae6) will increase coverage by 0.04%.
The diff coverage is 71.87%.

@@            Coverage Diff             @@
##             main     #143      +/-   ##
==========================================
+ Coverage   91.18%   91.23%   +0.04%     
==========================================
  Files          11       11              
  Lines        3847     3832      -15     
==========================================
- Hits         3508     3496      -12     
+ Misses        339      336       -3     

Flag	Coverage Δ
unittests	91.23% <71.87%> (+0.04%)	⬆️

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

Files Changed	Coverage Δ
trollimage/tests/test_image.py	100.00% <ø> (ø)
trollimage/_colorspaces.pyx	61.90% <68.14%> (-2.36%)	⬇️
trollimage/colormap.py	100.00% <100.00%> (ø)
trollimage/tests/test_colormap.py	100.00% <100.00%> (ø)

[coveralls]

coverage: 91.479% (+0.04%) from 91.435% when pulling 7d256a4 on djhoese:opt-colorspaces into 06e2ae6 on pytroll:main.

[djhoese]

@pnuu Looks like I was playing around with some dtype preservation in this PR too (specifically in colorize). I'll need to review this next week.