Support channel map for range of channels

[pford]

Description
Draft PR pending frontend support for testing.

• What is implemented or fixed? Mention the linked issue(s), if any.
  Implements channel map view (backend) #1186
• How does this PR solve the issue? Give a brief summary.
  Using the channel_range added to SetImageChannels message, backend sends the requested tiles for the requested channels. The downsampled data for each channel is retrieved to create the tiles but is not stored. (TODO: check tile cache)
• Are there any companion PRs (frontend, protobuf)?
  channel map view (frontend) carta-frontend#1203 (issue), Add fields and message for channel map carta-protobuf#92
• Is there anything else that testers should know (e.g. exactly how to reproduce the issue)?
  Select channel map view, should be shown a grid of images with consecutive channels.

Checklist

• changelog updated / no changelog update needed
• e2e test passing / corresponding fix added / new e2e test created
• ICD test passing / corresponding fix added / new ICD test created
• protobuf updated to the latest dev commit / no protobuf update needed
• protobuf version bumped / protobuf version not bumped
• added reviewers and assignee
• added ZenHub estimate, milestone, and release

[github-actions]

Package	Line Rate	Health
src.Cache	72%	➖
src.DataStream	44%	➖
src.FileList	67%	➖
src.Frame	35%	❌
src.HttpServer	42%	➖
src.ImageData	28%	❌
src.ImageFitter	83%	✔
src.ImageGenerators	43%	➖
src.ImageStats	75%	✔
src.Logger	37%	❌
src.Main	52%	➖
src.Region	69%	➖
src.Session	3%	❌
src.Table	52%	➖
src.ThreadingManager	67%	➖
src.Timer	85%	✔
src.Util	37%	❌
Summary	45% (8638 / 19117)	➖