Check order of user-added variables for Nek wrapping

[aprilnovak]

With NekRSMesh, we create a mesh which is either HEX8 or HEX27 - if the user adds their own AuxVariable which is of higher-order than the NekRSMesh, MOOSE will throw an error. However, it seems like this error only ever gets propagated to the console if the user runs the cardinal-opt -i nek.i directly -- i.e., if Nek is a sub-app, they just get an unhelpful seg fault.

We should check that any user-added variables are compatible with the order of the NekRSMesh.