where MyNonlinearityType is MyNonlinearity<dimension, Function>

In doing this,
 * Make directionalDerivative() assume |dir| = 1 and
 * Kill pseudoDirectionalDerivative

Does not seem to work with out-of-source builds

Necessitates implementation of directionDomain

In most cases (x != 0) this is a lot faster than normalising the direction and computing the actual directional derivative