Code tracking for multipoles in a bend.

Multipoles can be either "horizontally pure", "vertically pure" or "unknown". I suggest handling unknown multipoles the same as multipoles for a straight element.

Start by locating the code that tracks multipoles through bend elements and compare it with the handling for multipoles in straight elements. Verify that an unknown multipole in a bend follows the straight-element behavior, and add or update coverage if an existing tracking test area is found.

Written by the indexing model from the issue text.