Keep a consistent labeling of boundaries for 2D periodic meshes

[connorjward]

This PR changes the labels of 2D periodic meshes so that they always resemble

    4
 +-----+
 |     |
1|     |2
 |     |
 +-----+
    3

regardless of periodicity (some labels can be empty). This contrasts with the current solution where the boundary labels are

    2
 +-----+
 |     |
 |     |
 |     |
 +-----+
    1

or

 +-----+
 |     |
1|     |2
 |     |
 +-----+

depending on the direction of periodicity.

This is useful to do because:

• It's more intuitive, the old solution only existed because of how we generated periodic meshes via higher-dimensional manifold meshes.
• It lets us discard some tricky hacks

This is unfortunately and unavoidably an immediately breaking change but we at least warn users if they try to use an empty boundary set.