Unify and fix BoundingBoxtree constructor (C++)

[jorgensd]

Reported by @adeebkor.

The constructor:

 /// Constructor
  /// @param[in] mesh The mesh for building the bounding box tree
  /// @param[in] tdim The topological dimension of the mesh entities to
  /// build the bounding box tree for
  /// @param[in] padding Value to pad (extend) the the bounding box of
  /// each entity by.
  BoundingBoxTree(const mesh::Mesh<T>& mesh, int tdim, T padding = 0)
      : BoundingBoxTree::BoundingBoxTree(
            mesh, tdim, range(mesh.topology_mutable(), tdim), padding)
  {

does not work at the moment (and is never tested), as range takes in a reference, not a shared pointer.

To fix this, and follow the rest of DOLFINx, I've unified the constructors using std::optional.

This also removes the None-conversion in the Python layer.