Massive spatial fields

Anari requires `data` for a `structuredRegular` spatial field to be supplied as an array. This precludes implementations that support rendering massive volumes by streaming data from disk.

I could see this being addressed by allowing `data` also to accept a `STRING` indicating the path to data, where responsibility is left to the implementation to retrieve the data. EDIT: Or a new type of spatial field. Of course, there are many volumetric data file formats, and realistically, there would be little overlap between the formats supported by implementations (if they even support any), so I don't know if it is a good fit for the ANARI API. Thoughts?

Start with the ANARI structuredRegular spatial field definition and review how its data requirement is described. Compare the proposed path-string input with the alternative of a new spatial-field type, including the implications of implementation-specific volumetric formats. Done means reaching and documenting a settled API direction.

Written by the indexing model from the issue text.