Rename \U to \unit in wtk_cmmds.sty.

Rename \U to \unit in wtk_cmmds.sty.

I ran into a collision while working on my thesis:

  ! LaTeX Error: Command \U already defined.
                 Or name \end... illegal, see p.192 of the manual.
  ...
  ! LaTeX Error: Command \U unavailable in encoding OT1.

Debugging with `\show\U`, I got:

  > \U=macro:
  ->\PU-cmd \U \PU\U .

which seems to be something relating to fonts.  The new implementation
still defines \U, but only if nobody else has beaten us to the punch.
If they have, but you want \U to mean "unit", you can add

  \renewcommand{\U}[1]{\unit{#1}}

after both the colliding package and wtk_cmmds.sty have been imported.
This might break the other package, but hopefully they won't be
relying on such a collidable name for critical internal functionality.