Rename operator property "symmetric"
Strictly speaking, the property of EndomorphicOperator (and children) called "symmetric" does not reflect the symmetry of the operator matrix, but rather whether it is self-adjoint/hermitian. (Otherwise, DiagonalOperator could always just return "True" for this property.)
I'd suggest to rename this property to "self_adjoint" or "hermitian". If there is agreement, I'll provide a merge request.