GRHydro: add check when ADMMacros::spatial_order is used

This patch tries to catch isues where one uses an old parfile that sets ADMMacros::spatial_order but does not set GRHydro::sources_spatial_order. It is a best a help since if one actually disables the ADMMacros thorn (which is no longer required) then there is nothing that the patch can do to catch issues like in https://trac.einsteintoolkit.org/ticket/2119 and https://trac.einsteintoolkit.org/ticket/2118

Ticket is here: https://trac.einsteintoolkit.org/ticket/2120