Feature request: Add 'subset selection' quantities for vertices/faces/points/etc

[nmwsharp]

Add quantities that track a subset of vertices/faces/points on a structure. Allow the user to interactively update the selection, and add a function to query the current selection.

This sliiiiightly violates Polyscope's 'one-way visualization only' principle, but I think it is okay. This is a quite common need.

[the13fools]

Another feature request along these lines, add native support for selecting/querying by id instead of needing to click around/manually dump state to command line.