🆕Multichannel Image Reading

[measty]

Initial implementation of multi-channel reading.

The core tasks are done, the others can be a later PR if we decide they are necessary.

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
• Third set of metadata format for COMET
• For better usability join the menus for channel and colour (so not scrolling separately)

[codecov]

Codecov Report

❌ Patch coverage is 99.77629% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.41%. Comparing base (78b797e) to head (ae7e2ca).

Files with missing lines	Patch %	Lines
tiatoolbox/visualization/bokeh_app/main.py	98.07%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #825      +/-   ##
===========================================
+ Coverage    99.37%   99.41%   +0.03%     
===========================================
  Files           71       72       +1     
  Lines         9175     9540     +365     
  Branches      1197     1267      +70     
===========================================
+ Hits          9118     9484     +366     
+ Misses          31       29       -2     
- Partials        26       27       +1     

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[shaneahmed]

This looks good and I can successfully load the images in TIAViz. However, I am not clear how to read the channels in python. Could you add some examples in the docstring?