Class to attach basic material data to `mvdf_obj` objects

Slots

x,y,z: Numeric: distance of the origin of the object from the origin of the grid system (the central point at 0, 0, 0) in meters in the given direction. Must have no `NA`, `NULL`, `NaN`, `Inf`, or `-Inf` values. Coordinates are assumed to be on a right-handed coordinate system with Z oriented as the natural "vertical" direction.
idx: Character: a unique identifier (or "index") for each object to be modeled. Must be unique with no `NA` or `NULL` values, but otherwise is not validated.
metadata: Data frame: a table containing additional information on the objects to be modeled. Optional, but if this slot is used then the data frame must contain a column named `idx` which should correspond to the `idx` slot. Only the existence of this column is validated.
appendix: List: additional data produced in the generation of the object. Not validated; any additional outputs that don't map to modeled objects may be inserted here.
diffuse_color: Diffuse color of the material, as an RGBA array of floats scaled from 0-1.
metallic: Amount of mirror reflection for raytrace, as a float from 0-1.
roughness: Roughness of the material, as a float from 0-1.