Only define \U{} and \E{} workarounds in wtk_graph.asy.

Only define \U{} and \E{} workarounds in wtk_graph.asy.

We need redefinitions beacause asymptote missaligns labels if we use
\U{} and \E{} directly.  However, lots of \mbox{} and \cdot10^{} cruft
is both ugly and brittle.  The reworked units() and new Exp() should
be used instead.

math(string) is a simple replacement for "$"+string+"$" which looks
clearer to me in most cases, so I've added that too.