Make TableCoordinates support pixel edges and centers

[DanRyanIrish]

PR Description

This PR enables support for custom pixel edges in QuantityTableCoordinate and TimeTableCoordinate objects by storing the underlying grids as including pixel edges and centers, not just centers as heretonow.

The motivation is that world coordinates at pixel edges are often wanted. When the pixel widths are uniform, the edges can be found from the centers. However, when this is not the case, explicit knowledge of the edges is required.

[DanRyanIrish]

@Cadair Can you advise on whether this is the best way to go about this?

[DanRyanIrish]

Make TableCoordinate require the input pixel grid to which the input world grid corresponds, and then keep that as the underlying storage. Enable shortcuts for "center" and "corners".

[github-actions]

Hello 👋, Thanks for your contribution to ndcube!
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[github-actions]

Hello again 👋, We want to thank you again for your contribution to ndcube!
This pull request has had no activity since my last reminder, so I am going to close it. If at any time you want to come back to this please feel free to reopen it! If you want to discuss this, please add a comment with: @sunpy/ndcube-developers and someone will get back to you soon.