Additional functionality for ProductManifold

[johnharveymath]

The default representation for a product manifold is a vector of points made by flattening and concatenating points from the individual factors. At runtime, for many methods, this vector is decomposed and reshaped into the individual points, acted on, and then reflattened.

However, many users may actually prefer to represent the points as lists and receive outputs as lists. It should be possible to add functionality to methods that take arrays by allowing them to alternatively take a list of n_factors arrays. This would be more efficient as the internal methods which slice and dice the arrays could be skipped.