Generalize the data structures for the cells adjacent to an edge.

Generalize the data structures for the cells adjacent to an edge.

In 2d, only two cells can be adjacent to an edge, and the current data structures in the
mesh reorientation algorithm reflect this: each edge had 2 slots for adjacent cells, of which
zero, one, or two could be used at any given time.

On the other hand, in 3d, an arbitrary number of cells may be adjacent to an edge. I eventually
want to extend the algorithm re-written in #3166 to the 3d case as well. This patch generalizes
the data structure representing the cells adjacent to an edge by making it generic. The patch
only implements the 2d case, but the 3d case will be easy to add later on, and it makes the 'Edge'
data structure generic.