Fix VelocityPenalty, add a regression test

So it turns out that my "eyeball norm" tests were worthless, and so
was the old VelocityPenalty.  This test will do as a stopgap and
should be easy to replace with something more analytic when I find
time.
Merge pull request #124 from grinsfem/fix_velocity_penalty

Fix VelocityPenalty, add a regression test