Example of how to use time-dependent covariates

Hi, thanks for the repo!

It looks from [the article](https://arxiv.org/pdf/1907.00825.pdf) that CoxTime can do time-dependent covariates. Is this functionality implemented in pycox? If so, do you have any examples on how to format the inputs to the model for this usecase?

Thanks again,
Guillem

Start by checking pycox's CoxTime implementation against the linked article and look for existing input-format examples. Document whether time-dependent covariates are supported and show the required inputs if they are; the issue provides no file or test path.

Written by the indexing model from the issue text.