Support for the `tensor` package

[JeppeKlitgaard]

The tensor package in LaTeX is quite handy for writing tensors with mixed covariant and contravariant indices in an ergonomic fashion.

I think the package is quite small and would be fairly easy to add. I doubt I will get around to doing it myself, but there is a small change that it could happen. Would a PR for this package as a first-party extension be accepted should I ever get around to it?

[dpvc]

It does look like it would not be too bad to implement, and might be a good first project for someone interested in contributing to MathJax. We would certainly entertain a PR for such an implementation, thought that is not a guarantee of acceptance.

[GaugeAndGravity]

Hay is anyone still working on this? I really need this feature, but I don't know how to do this yet