Remove pyvista dependency and update export options

[Carifio24]

This PR removes the package's dependency on pyvista, which resolves #32. This involves a large refactor of the package and involves a few major changes:

• Remove support for exporting to OBJ files
• Add support for exporting to USD files
• Add voxel-based export (in addition to isosurface method) for volume layers
• Add meshoptimizer as a glTF compression option
• Use registry-based system to add layer export methods
• Update the export dialog to allow for selecting a different export method (if available) for each layer
• Improve sizing of spheres for scatter layers