check everything am what they is

[mdsumner]

I.e. plot3d(<linear topology>) is always linear, but persp3d(<linear topology>) has a crack with TRI (PATH) or DEL (SC) and sends out a message: "linear topology input but 'persp3d' implies surface topology, so we ".

[mdsumner]

also need checks for modifying models, currently copy_down(SC0, z) returns SC ... which would be improved if all the getters were functions, and so we could get_topology_(SC) by simply wrapping it and returning the structural indexes as if they were present.