Consolidate Edge Velocity Features

[ddmarshall]

The 3 edge velocity functions/variables need to be replaced with a class that can assume the responsibility of creating the missing derivatives and other things that the current set_velocity() method does. This should make the code cleaner, help with code cohesion, and simplify the ibl_method class.

[ddmarshall]

The edge velocity class should have from_*() methods that can construct a velocity object from the various ways that the velocities can be constructed.