Compute simple version of center and radius of 2 meshes.
surfs.props(lh, rh)
`fs.surface` instance, the mesh for the left hemisphere. If a string, assumed to be the path to a surface mesh file that should be loaded.
`fs.surface` instance, the mesh for the right hemisphere. If a string, assumed to be the path to a surface mesh file that should be loaded.
named list with entries 'center', 'radius', 'min' and 'max', each of which are numerical vectors of length 3, holding the x, y and z coordinates or values.