Updates to inspector plot: add rectangle mode and remove operation arg

[nvaytet]

Some updates to the inspector plot:

• fixed slicing when used on data with a 2d coordinate
• small optimization to avoid converting coords to midpoints every time a polygon is modified
• added rectangle analog to feat: polygon inspector tool #518

A question arose while implementing this: there is an operation argument that decides which reduction operation to apply along the 3rd dimension into the paper. This is a sum by default, but can be a min, max or mean.
Now, when we select a region inside a rectangle or a polygon, we need to also reduce that along the x and y dimensions before sending to the 1d plot along z. The operation there is currently always a sum, and I don't think it would make sense to do anything else than sum?

So the question is: does it even make sense to have other operations along the z dimension?

Update: I decided to remove the operation in the end.

[nvaytet]

TODO: still need to write some tests

[nvaytet]

I don't think we need to convert the unit here? By construction, the xy locations returned by the polygon info are in the correct canvas units.