[d3] Error message when adding/multiplying S2 bases with different radii is not helpful

[jsoishi]

If you accidentally try to add two S2 bases with different radii, the error message is

TypeError: unsupported operand type(s) for +: 'SphereBasis' and 'SphereBasis'

This does not explain the nature of the error. Given this is a very easy error to make, particularly when setting boundary conditions in spherical shells, it would be good to at least make a suggestion of checking radii.