Eliminate Extents and Position classes

The Extents and Position classes are redundant with ImgLib2 classes. The Position class may be equivalent to a Cursor. We may be able to eliminate them in favor of Intervals.

Migrated-From: http://trac.imagej.net/ticket/1996

Start by locating the Extents and Position classes and reviewing how they relate to the ImgLib2 Cursor and Intervals concepts mentioned in the issue. Determine the replacement scope and affected callers before deciding whether both classes can be removed; done means the redundant classes are eliminated without leaving unresolved uses.

Written by the indexing model from the issue text.