Averaging of velocities at U point for C and CD grids

[JFLemieux73]

We need to investigate whether we should have 2 or 4 point averages in strain_rates_U for calculating uvelU and vvelU.

[dabail10]

In the document, we need uNu, uEu, vNu, vEu which are the CD grid velocities interpolated to the U points. Currently we only interpolate the C-grid velocities to the U points.

[apcraig]

Does this have anything to do with #792?