Add utilities for building `UnitInput` from `CsgTree` (#1155)
* Add geo setup * Add find_sorted * Refactor `build_postfix` into a function that takes a remapping function * Further refactor postfix construction to split faces from logic * Explicitly use node type for csg tree size for documentation * Rename exterior volume name and add global universe * Add exterior bounding box helper * Move postfix builder to a separate file * Add internal surface mapper