Added support for monochrome images decoding

[bigcat88]

Changes proposed in this pull request:

• if image is monochrome, from now it will be ALWAYS opened in native mode, if it is 8 bit then it is L if HDR then I;16
• info["chroma"] now filled for images that contains such information, can be used during save
• updated Test Images with correct data, some of them L_8_29x...heic were saved not in monochrome mode.

If your code expects only RGB images as input, then from this release you need to convert monochrome to RGB, fortunately this is easy to do with Pillow.

There will be no support for monochrome images with the Alpha channel until Pillow starts supporting HDR images, in the current situation there is already enough unnecessary code just because Pillow does not support HDR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (9f4acd0) to head (df6f18f).
Report is 1 commits behind head on master.

Additional details and impacted files

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##            master      #215   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines          927       936    +9     
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+ Hits           927       936    +9     

Files	Coverage Δ
pillow_heif/as_plugin.py	100.00% <100.00%> (ø)
pillow_heif/heif.py	100.00% <100.00%> (ø)
pillow_heif/misc.py	100.00% <100.00%> (ø)