Sphere_centre [radius]
Choose atoms in a sphere centred at the current active centre. The
coordinates of this point are stored in the .active_centre
datablock.
If an atom from a residue is within the given sphere radius, all atoms in
that residue get chosen. The .active_centre datablock is set by any
of the commands Centre_ID, Centre_xyz, Centre_atom,
Centre_zone, or by the user directly overwriting the value stored in
the database.