More consistent io / load

[almarklein]

In Pygfx there's a few places where we support data to be loaded with external libraries. In particular:

• in geometries/_compat.py for converting external geometry to pygfx geometry.
• In materials/_compat.py for converting external materials to pygfx materials.
• In utils/load.py for loading whole meshes, scenes, etc.

As Korijn said, the important parts are:

• Ensure external lib imports are optional
• The compatibility layer between external objects and pygfx objects is what matters most
• Direct loading functions are for convenience
• Consistency between different libraries is a pipe dream

Let's not try to make a single abstraction over multiple external libraries. Instead I propose:

• The compat functionality for geometries and materials is fine.
• Create a load or io subpackage that is basically a collection of functions to load (and store?) stuff using external libraries.
• Give this a separate section in the docs (instead of as part of pygfx.utils).
• Remove all io from the main gfx namespace (deprecate first).

[almarklein]

basically a collection of functions

IO is inherently messy. By constraining it to a specific namespace, it's somewhat scalable without affecting the main pygfx API.

[Korijn]

There is meshio: https://github.com/nschloe/meshio