The function directional_diff(f, V, coord) computes the directional derivative of the scalar field f with respect to the coordinates coord in the direction of the contravariant vector V.

f must be of type algebraic, and V must be a tensor_type with index character [1] (a contravariant vector field).  The result is an algebraic expression.

Simplification:  This routine uses the `tensor/directional_diff/simp` routine for simplification purposes.  The simplification routine is applied to each component of the gradient of f (an intermediate calculation) and once to the contraction of this gradient with V (the result).  By default, `tensor/directional_diff/simp` is initialized to the `tensor/simp` routine. It is recommended that the `tensor/directional_diff/simp` routine be customized to suit the needs of the particular problem.