ENH: Remove Normal node. Use more general GaussianArrayARD instead.
ENH: Rename GaussianArrayARD to GaussianARD

This naming convention is used also in other places where
GaussianArrayARD appears, for instance, RotateGaussianArrayARD is now
RotateGaussianARD.

In the future, also Gaussian will be a node for arrays, not just for
vectors.