post-initialization add_variable() is unsupported
post-initialization add_variable() is unsupported

We might want to change that eventually, but for now let's just try to
catch it when it happens.

Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh