this allows for:
include <parameters.scad>
// override a variable such as
frame_z = 60;
include <components.scad>
which will let individual components tweak settings, e.g. the height of
a frame
this complicates the print, in that now printing the frame requires
supports, but by cutting in on the bottom the same way as the top, we
can center some stuff better and also make the plates a bit more
interchangeable. this also eliminates the slightly unsightly bottom
panel hanging out in the open, so overall I think it's worth the
printing complication
