Nuke obsoleted template parameter

If |u| is computed as 0, we should not divide by it (this appears to
happen even for u != 0)

The directional subdifferential is assumed to be taken with respect to
a coordinate system in which v is a unit vector judging by the
implementation of subDiff in directionalconvexfunction.hh

If we do not take that into consideration, subDiff() will mix input
from two different coordinate systems and return something meaningless

As long as we only care about the sign we need not perform any
back-transformation!

SmoothGrad -> smoothGradient
PlusGrad   -> upperGradient
MinusGrad  -> lowerGradient