Loading scenes from files at runtime

[Walther]

Currently, all the scenes are defined as code in the src/scenes directory as .rs files. To render a different scene, a whole recompile of the project has to happen.

It should be possible to run clovers --input scene and get a render of the specified scene, or helpful error messages pointing out mistakes in the scene file.

Open questions:

• which format? TOML? JSON? Other, what?
• what should a scene contain?
  • objects
  • camera
  • priority objects (used for probability sampling; e.g. lights, glass objects)
• optional future pondering: how to specify change over time, for rendering video / multiple frames?