Write converters for ArrayImg to primitive array types

From [a post by @dietzc on the ImageJ forum](http://forum.imagej.net/t/suggestions-for-iitorai-computerops/415/9):
`ArrayImg`s should have a `Converter` to the native array and then the ops framework can automatically select the most efficient implementation for native-array-friendly operations.

Start by locating ArrayImg and Converter implementations in the ImageJ common data model, then inspect how the ops framework discovers converters. Done means ArrayImg can convert to the relevant native primitive array types and the ops framework can select those implementations automatically.

Written by the indexing model from the issue text.