🆕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:

• [x] 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
• [x] Get colors from metadata (if existing)
• [x] Selecting specific channels in viewer
• [x] Third set of metadata format for COMET
• [ ] For usability join the menus for channel and colour (so not scrolling separately)

[codecov[bot]]

Codecov Report

:x: Patch coverage is 82.21709% with 77 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 98.51%. Comparing base (3242b1a) to head (165909b).

Files with missing lines	Patch %	Lines
tiatoolbox/utils/postproc_defs.py	52.72%	19 Missing and 7 partials :warning:
tiatoolbox/visualization/bokeh_app/main.py	66.66%	16 Missing and 2 partials :warning:
tiatoolbox/visualization/tileserver.py	34.61%	16 Missing and 1 partial :warning:
tiatoolbox/wsicore/wsireader.py	94.71%	3 Missing and 11 partials :warning:
tiatoolbox/models/dataset/classification.py	93.75%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #825      +/-   ##
===========================================
- Coverage    99.27%   98.51%   -0.77%     
===========================================
  Files           71       72       +1     
  Lines         9161     9518     +357     
  Branches      1195     1265      +70     
===========================================
+ Hits          9095     9377     +282     
- Misses          40       94      +54     
- Partials        26       47      +21     

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[shaneahmed]

This PR should be able to read output files from #929