Mesh subsets

This adds mesh subsets to the MeshLib. There is a `Location` struct describing a spatial location by a triple of mesh id, mesh item type and its local id.

There is a room for improvement regarding memory efficiency, especially if a mesh subset consists of a complete mesh.

This is also duplicating `isActive` flag on mesh items, but not substituting it. We shall discuss, if `isActive` flag is still needed (@norihiro-w ?)