Implement 'intensity' parameter in Surface as in Mesh3D

[gevero]

Usercase: when performing a 3D parametric plot as in these examples, surface coloring is performed as a function of z coordinate. It could instead be useful to add the possibility to perform the surface coloring as a function of a scalar field defined on the surface. An example could be performing the coloring as a function of the distance from the origin r=sqrt(x^2+y^2+z^2).

Solution: add an intensity parameter, as already present in Mesh3D, also analogous to the scalars parameters found in the mlab mesh function.

[etpinard]

@gevero Thanks for writing.

We've been planning on adding this feature for a while now. It should come online this spring.

[gevero]

@etpinard Thanks a lot.