Fix a bug where line manifold ids were not copied correctly.

Fix a bug where line manifold ids were not copied correctly.

When we convert hexes to tets, we create three types of lines:
1. lines inside the hex
2. lines on the faces of the hex
3. lines equal to the hex's lines

In the last case we need to copy the manifold and boundary data from the
preexisting lines directly, since they may not be equal to the ids assigned to
the adjacent face.