Build math taper inconsistent with box etc.

For the Box family (Box/Cylinder/Prism), the variables for taper and hole size have the wrong meanings.

If you use "XTP" or "YTP", it uses the values from the Torus family taper attribute. "XHL" and "YHL" (X and Y hole size) give the correct values for Box family taper.

This makes sense from an implementation perspective, since LL rather stupidly made Box's "taper" attribute and Torus' "hole size" attribute have the same slot. But from a UI standpoint, taper should mean taper all the time.

There's the issue of what the values for "hole size" should be for Box family, but I think it's reasonable to just have them evaluate to 0.