Fix dy2-vs-dxdz transposition
Fix dy2-vs-dxdz transposition

This has been sitting here broken ever since @pbauman broke it in 2012.

I ... probably should have written unit tests sooner.

Fortunately all my codes were using the get_d2phi() call rather than
the component-by-component calls.

libMesh/libmesh,capitalaslash/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,libMesh/libmesh,dschwen/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,libMesh/libmesh,roystgnr/libmesh,capitalaslash/libmesh,libMesh/libmesh,dschwen/libmesh,jwpeterson/libmesh,roystgnr/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,roystgnr/libmesh,jwpeterson/libmesh,roystgnr/libmesh,libMesh/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,dschwen/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,dschwen/libmesh,libMesh/libmesh,libMesh/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,roystgnr/libmesh,dschwen/libmesh,BalticPinguin/libmesh,dschwen/libmesh,libMesh/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,dschwen/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,dschwen/libmesh