Implement CircleSectorPixelRegion

[adonath]

This PR is a draft to implement a CircleSectorPixelRegion. I needed this for a specific analysis, so I decided to just implement it. Here is a notebook to illustrate that the bounding box and contains methods work correctly for all quadrants https://github.com/adonath/notebooks-public/blob/master/circle_sector_pix_region_illustration.ipynb

Some ToDos:

• Implement a CircleSectorSkyRegion
• Implement CircleSectorPixelRegion.to_sky()
• Implement CircleSectorPixelRegion.to_mask()

[adonath]

@larrybradley is this a desired feature? Are there any guidelines with respect to adding new shapes to regions?

[larrybradley]

@adonath This looks like a nice addition to me! Implementing the "exact" mode for exact partial pixel overlaps could be tricky.

[jzuhone]

I'd love to help with this. What do we need to get it across the finish line?

[keflavich]

@adonath are you still working on this? If the example works, maybe we can just merge (after resolving conflicts) and leave the unimplemented modes as open issues?

[adonath]

@jzuhone @keflavich I have not made any progress since, but I would be happy rebase and open issues for the remaining tasks.