Derivate of Kernel function wrt lengthscale applied on inputs X and X2.
		In the stationary case there is an inner function depending on the
		distances from X to X2, called r.

		dKd_dLen(X, X2) = dKdLen_of_r((X-X2)**2)

def dKd_dLen(self, X, dimension, lengthscale, X2=None):
		"""
		Derivate of Kernel function wrt lengthscale applied on inputs X and X2.
		In the stationary case there is an inner function depending on the
		distances from X to X2, called r.

		dKd_dLen(X, X2) = dKdLen_of_r((X-X2)**2)
		"""
		r = self._scaled_dist(X, X2)
		return self.dKdLen_of_r(r, dimension, lengthscale)