Update to changes in Base's cov and cor

[andreasnoack]

These changes are necessary after JuliaLang/julia#13465 where the keyword arguments for mean and vardim were removed. Notice that the tests there won't pass before JuliaLang/julia#13505 has been merged. I got some of the deprecation definitions wrong in the first attempt.

[tkelman]

The new syntax should be added to Compat, this won't be the only use case.

[andreasnoack]

I wouldn't be so sure about that, but they are in progress anyway. I think few people uses a custom mean or MLE corrections when computing covariances. I tried to search through julia repositories on GitHub, but unfortunately the search doesn't allow = in the search expression so it's quite difficult for seach for the use of keyword arguments.