rotate function for prism objects

[oskooi]

It would be useful to add a rotate member function to the Prism object with a single argument theta that is used to rotate all the vertices in a plane perpendicular to axis. This feature should also work with get_GDSII_prisms. A possible test could involve importing a ridge waveguide from a GDS file and verifying that the Poynting flux for its fundamental mode is the same at different rotation angles (similar to what was demonstrated in #1130).