Cycles : add a Volume Geometry Node

This splits the volume related data (properties for rendering and attributes) of the Mesh node
into a new `Volume` node type.

This `Volume` node derives from the `Mesh` class since we generate a mesh for the bounds of the
volume, as such we can safely work on `Volumes` as if they were `Meshes`, e.g. for BVH creation.
However such code should still check for the geometry type of the object to be `MESH` or `VOLUME`
which may be bug prone if this is forgotten.

This is part of T79131.

Reviewed By: brecht

Maniphest Tasks: T79131

Differential Revision: https://developer.blender.org/D8538

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