Quality-of-life improvements

[klarh]

Ideas from discussion with @bdice:

Automatically zoom (more appropriately, set size and pixel_scale) to fit the primitives contained in a scene

Solutions:

• Add an auto_zoom() method to Scene
• Add machinery to zoom property setters to auto-zoom when setting prim.zoom = 'auto'

Add a way to access commonly-used orientations (isometric/viewing each face/slightly-off-each-face)

Solutions:

• Add a plato.rotations module that contains these by name
• Add machinery to rotation property setters to convert string descriptions into appropriate quaternions

Easier way to access child primitives via Scene objects

Solutions:

• Add __getitem__ to scenes that searches through the set of contained primitives for the given key (something like 'spheres'/'convex_polyhedra' -> the first primitive of the given type, 'spheres_2' for the second spheres object in the scene, and so on? Also allow querying by draw class/type?)

[bdice]

Rotations

Some helpful rotations to include:

• Front, left, top, right, rear, bottom
• Isometric view for each corner ('isometric' would default to something like top, front, and left).
• "Offset" view(s) that show mostly the front face but shows some of the top and side faces.
• I would write these using rowan's axis-angle formula for readability, rather than using explicit constants

I don't think the string setter mechanism is strictly necessary, but it could be helpful.

Accessing child primitives

I think it's most natural for __getitem__ to require integer indices, and potentially have a function like scene.find_primitives(query) for the other case, if needed. I would be happy to just have an integer index, no fancy querying by primitive type is needed in my opinion.