Added support for full rank yerr, not just diagonal.

[ixkael]

Added support (and tests) for full yerr, not just diagonal. Happy to add more tests.

[dfm]

Thanks Boris! This is interesting and I can see the use but I don't think that it's right to call this "yerr" because it's more like the measurement "covariance". I'd prefer to add it as a separate parameter - maybe a kwarg called cov or something.