Componentwise Operator

[pmli]

Models with componentwise nonlinearities, i.e., functions $f : \mathbb{R}^n \to \mathbb{R}^n$ of the form $f(x) = (g(x_1), g(x_2), \ldots, g(x_n))$ for some scalar function $g$, seem to be common enough to justify adding a ComponentwiseOperator (or named differently) to represent $f$ using $n$ and $g$.

This would be an application of #765.