🆕Multichannel Image Reading

[measty]

Initial draft implementation of multi-channel reading.

Still needs lots of work so its a draft at the moment.

to do:

• test on other examples
• unit tests
• decide what should default behaviour be?
• decide how will interact with VirtualWSIReader mode flag - should we extend this to all readers?
• are there better ways to do rgb conversion? simple approach at the moment
• Get colors from metadata (if existing)
• Selecting specific channels in viewer
• anything else?

[codecov]

Codecov Report

Attention: Patch coverage is 53.15315% with 52 lines in your changes missing coverage. Please review.

Project coverage is 99.29%. Comparing base (14f1215) to head (83c6954).
Report is 4 commits behind head on develop.

Files	Patch %	Lines
tiatoolbox/wsicore/wsireader.py	50.00%	24 Missing and 20 partials ⚠️
tiatoolbox/utils/postproc_defs.py	61.90%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #825      +/-   ##
===========================================
- Coverage    99.89%   99.29%   -0.61%     
===========================================
  Files           69       70       +1     
  Lines         8650     8737      +87     
  Branches      1653     1686      +33     
===========================================
+ Hits          8641     8675      +34     
- Misses           1       32      +31     
- Partials         8       30      +22     

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