LEGS _XZ component is now parsed

It turns out that it didn't matter what values you put into
the leg definition for the XZ component (the last one) because
the slices done were off by one. That has been fixed. However,
in digging around to diagnose and fix this problem I have found
some crazy instability where prescribed strains get a mind of
their own in some situations.

I also added another analytical benchmark. It's really too bad
that the tolerances on it are so low because of implementation
problems. Sometime when I have more time I'll likely fix it.