This function calculates profiling velocity from depth and time and adds the values to the
data field in the RSK structure. This function requires an RSK structure that contains depth (see
RSKderivedepth). The depth channel is smoothed with a running average of length
windowLength to reduce noise, and then the smoothed depth is differentiated with respect to time to obtain a profiling speed.
If there is already a velocity channel present,
RSKderivevelocity replaces the velocity values with the new calculation.
If there is not a velocity channel but depth is present, the function adds the velocity channel metadata to
channel field and calculates velocity based on the values in depth column.