Adds a stochastic parameterization of unresolved/tidal currents

- The CM4 model has a slow sea-level fall because mass is being locked
  up in bergs trapped in single point inlets and channels. This happens
  because the winds are masked out on land cells.
- This commit adds random vectors to the ocean currents in the vicinity
  of land cells so that particle dispersion can allow the bergs to
  eventually escape. The controlling parameter is a velocity amplitude.

Author: adcroft