allow zero height for objects in 2d?

[stevengj]

It would be nice to allow objects to have zero thickness in 2d simulations, especially prisms since that is the default when loading GDSII files (see also NanoComp/meep#1196).

[stevengj]

It looks naively like the prisms check 0 ≤ z ≤ height, which would allow zero height, but maybe there is a rounding error occurring.