[jlee-kitware]

Title: Fields evaluated where they are read, declared on Space

Split from #300. Stacked on #357, with #355 merged in.

Problem. A space can compute its field where it is read instead of storing values, as a field expression does. #300 supported that by probing undeclared attributes with getattr, and nothing in Tack tested those paths.

Approach. Space declares six attributes, each with its default:

• evaluated: the view’s value and corner_value compute the field, and its values hold nothing to read.
• continuous: contour shares crossings between cells. It is true for H1.
• default_subdivision: how many lattice steps per cell axis contour, clip and threshold cut a cell into by default.
• integration_order: the polynomial order integrate’s quadrature is exact to. integrate refuses an evaluated space that gives none.
• rebind(field, data): the field made again on a filter’s output, or None, in which case the filter leaves it out.
• stored(field, data): the field’s values, for the reductions, owned_values and dataset_to_vtk.

The algorithms read these directly, and the reference docs have a table of them. threshold now reads an evaluated field at its lattice points instead of treating it as cell data. An unbounded value_range makes clip and threshold cut every cell, so they return the same region in more pieces.

Verified. A toy space in the tests computes x*y + z where it is read. It covers contour, clip, threshold, carry and rebind, integrate (including refusing a space with no order), statistics, entropy, owned_values and the writer’s stored values. Each is compared with the same function stored in order-2 H1, which holds it exactly on these hexahedra.