Conditional velocity ghost update

[drwells]

Fixes #143, #145, #146. Part of #142.

With a little work (maybe two hours?) we can make, for the Turek-Hron benchmark:

• interpolateVelocity() about 15% faster
• spreadForce() about 10% faster
• forwardEulerStep() and mindpointStep() about 3x faster

just by avoiding some copies and being more careful with ghost updates.