ZvsXY: add series of points above the surface

Given a n-length Dataset with variables x, y and z, ``ZvsXY` plots z vs x and y as a surface based on triangulation. It can also add the n points above this surface. For various reasons, one may want to add other series of points above the surface. For instance, the surface and the first series could be built from model evaluations while a second series could be a set of experiments.

Proposal

Add an option other_datasets: Dataset | tuple[Dataset, str] | None = None where tuple[Dataset, str] corresponds to both dataset and color.