The bottom of the board by default always lands at z=0. The top is the
thicknesses of all the substrate layers added together. It is only the
substrate layers because right now the copper is just a change in
attribute on the surface of the substrate (2D) not a 3D object.

So for the hairpinfilter example which is just 1 substrate layer of
1.5mm. It would be z=0mm for the bottom, and z=1.5mm for the top.

That adjustment we have for z-mesh "num in substrate" is meant to
select how many lines fit inbetween those two lines.

Technically my pcb2csx library can do both full 3D where all objects
have real thickness and the thing we do now but it eats a lot of
processing power for marginal benifit in most cases. In other words
it's not worth supporting at this point.

