Problems with periodic BCs

I ran into two issues when using `PeriodicDirichlet` which I want to document here: 1. They are not affected by `update!` When you know that `AffineConstraint`s are not updated, this is not surprising, but in my opinion it would be good to have it updated. (I wanted to use that to solve the same system with different inhomogeneities.) Since @kreutzheidephil wanted to take a look at the `AffineConstraint`s, he might be able to solve this. 2. When defining the boundary, the approaches `∂Ω = ["left" => "right", "bottom" => "top"]` and ``` img = union(getfacetset(grid, "left"), getfacetset(grid, "bottom")) mir = union(getfacetset(grid, "right"), getfacetset(grid, "top")) ∂Ω = collect_periodic_facets(grid, mir, img) ``` Do not give the same result (for the first version weird things are happening at the corners). Again, I am in contact with @kreutzheidephil to find a solution, but wanted to document the problem here.