Use a more stable sign on VelocityPenalty force

We were antidissipative before; we should be dissipative now.

This commit also has the beginnings of the code for a term with no
energy addition *or* subtraction, but that may not be working right
yet.
Adjust penalty_poiseuille regression test

This should pass with the new VelocityPenalty sign.  Solution still
looks good to the eyeball but has some small numerical diff.
Merge pull request #149 from grinsfem/velocity_penalty_fixes

Velocity penalty fixes