Make Eval Mappable

Requires #397

Applying `Eval` to an image would ideally simply be a matter of mapping it. It would be nice if we could tie in the available parameters to the type of map (e.g. have a succinct way to express equations that query neighborhood values, which would trigger a neighborhood map being used).

Start by reviewing prerequisite issue #397 and the existing Eval and map concepts referenced here. Determine how image evaluation should be represented and how parameters would select a map type; the issue does not define specific files, tests, or an acceptance condition, so the design and completion criteria need maintainer clarification.

Written by the indexing model from the issue text.