Eliminate metric acts on non-tensors.

Calling eliminate_metric on expressions which are not actually tensors still raises/lowers the indices, e.g.

{m,n,o}::Indices. \partial{#}::PartialDerivative. g_{m n}::Metric. g^{m n}::InverseMetric.

\partial_{i}{g_{m n}} g^{n o}; @eliminate_metric!(%);

Fix requires associating a metric wrt. a derivative, e.g.

\partial{#}::PartialDerivative(metric=g). \nabla{#}::Derivative(metric=g).

etc.