Assimilation of surface velocity into a continuity scheme is now working
reasonably well. Several important changes include:

1. Modifying the physics to accept direction data.
2. Smoothing the merged direction data and steepest descent data.
3. Incorporating the errors into the bv_data_assim_errors.py script.
4. Applying a 'shape function' to convert the surface velocity to vertical avg.
5. A slightly refined mesh to avoid mismatches at the coast.

It should 'just work', although tuning will be needed. Of particular interest
will be:
1. Sensitivity to accumulation errors.
2. Possibilty of adding observed dHdt to initial values of 'apparent accumulation'.
3. Appropriate smoothing radius for the surface gradients. 14 seems large.