🆕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
• [x] Get colors from metadata (if existing)
• [ ] Selecting specific channels in viewer
• [ ] anything else?

[codecov[bot]]

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 :warning:
tiatoolbox/utils/postproc_defs.py	61.90%	6 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ 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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