use the agnostic "data" covmat instead of "t0" or "experimental" covmat to avoid confusion

[RoyStegeman]

Currently in the code the t0 covmat is sometimes also called "experimental covmat", while the published papers clearly distinguish t0 covmats from experimental covmats. In some places this is a result of the fact that the covmat is delivered through vp while the code that uses the covmat might be agnostic to the exact definition.

The proposed solution is to replace the names in the code and docstrings with "data" covmats, which can refer to either experimental or t0 covmat (or any other possible covmat in the future).

[Zaharid]

Makes sense to me.

[scarlehoff]

Did we ever do this? Do we want to do it?

[RoyStegeman]

No we never did, and I would no longer want to do it this way. Instead we should just use t0 and exp correctly (which I think may not be the case now for the docs)