Our operator overloading expression templates don't work with the GPU

[jonwong12]

I'm not sure if we noted it explicitly, but our operator overloading expression templates don't work with the GPU (mfem's memory management model makes it difficult to reason about what those expressions should do).

Originally posted by @samuelpmishLLNL in https://github.com/LLNL/serac/pull/523#discussion_r689815620