Add multichannel viewer [ENH]

[rogertrullo]

Hi, I see that we now have some readers that read images with multiple channels e.g., more than 3.
I was wondering if we could improve the TileServer viewer, so that we can use it for those images.
Basically we would need to convert the multi channel tiles to RGB, for example by specifying an RGB color for each channel.

[measty]

Hi roger,

Thanks for the suggestion! This may align with some other work i'm looking at doing on the tileserver, so if I get a chance to work this in I will do, and let you know when there is a relevant PR

[John-P]

Just tagging #610 as it is related to this issue

[measty]

This is being added in #825