Metrics wrapped in accents should only raise/lower indices on other accented objects

[kpeeters]

The example

{\mu,\nu,\rho}::Indices(vector, position=fixed).
g_{\mu\nu}::Metric.
g^{\mu\nu}::InverseMetric.
g_\mu^\nu::KroneckerDelta.
g^\mu_\nu::KroneckerDelta.
\delta{#}::Accent.

ex:=\delta{g_{\mu\nu}} g^{\nu\rho};
eliminate_metric(_);

should make Cadabra realise that the metric \delta{g_{\mu\nu}} cannot be used to lower the \nu index on the g^{\nu\rho}, because the latter is not also an accented object. The g^{\nu\rho} inverse metric can be used to raise an index on the first object.

See https://cadabra.science/qa/2920/question-about-eliminate_metric for the original question/discussion.