sampling interface additions

[cyrush]

For uniform grid:

• Allow spatial extents to be specified as an alternative to origin and spacing. This from will be more natural for many use cases
• Allow fields in addition to field, and support sampling multiple fields.

[cyrush]

Expand sampling cases beyond uniform grids, with a generic "sample" filter.

We should provide an interface that mirrors clip (perhaps expanded to also include the seed patterns in the streamline filter)

• box (this basically is our uniform grid filter)
• line (not in clip, but perhaps should be)
• plane
• sphere
• cylinder
• mesh: allow users to provide a blueprint mesh to sample onto

Like current sampling - start with results only on rank 0 (this is the most pressing use case)

[cyrush]

@nicolemarsaglia I updated this ticket with ideas about expanding how we leverage sampling to provide users more options.