Support xTensor notation

[eschnett]

One way to derive tensor equations is to use xTensor. This uses a slightly different syntax for abstract indices, symmetries, and to declare tensors. It would be nice if Kranc was compatible with it, or if there was a way to automatically translate xTensor output to Kranc.

[ianhinder]

Barry already implemented this, but it has not been fully tested. See #26. There might be some cases which don't work. I thought that there was an example in the Examples directory, but I can't find it. I have not used it myself. We should have an example of how it can be used, and we should add documentation for the feature once it is working well enough.