Allow floating-point isovalues for contours of integer fields

The flying edges version of the contouring filter converted the isovalues provided into the same type as the field. This is fine for a floating point field, but for an integer field the isovalue was truncated to the nearest integer.

This is problematic because it is common to provide a fractional isovalue (usually N + 0.5) for integer fields to avoid degenerate cases of the contour intersecting vertices. It also means the behavior changes between an integer type that is directly supported (like a signed char) or an integer type that is not directly supported and converted to a floating point field (like potentially a char).

This change updates the worklets to allow the isovalue to have a different type than the field and to always use a floating point type for the isovalue.

Backport: release