Explicit support for Symmetric from tn.Tensor.

[alewis]

Currently Tensor does not explicitly support the Symmetric backend. We should think about how to add such support.

• There's an argument to be made that symmetric tensors differ significantly enough from their other counterparts to warrant them forming a separate subclass, so that the charge and flow information is accessible (for instance).

• Initialization is different.

• Many operations should, nevertheless, pretty much work as written with Symmetric - it's just that I don't know which ones.