Text cca labels and centroids

If a user supplied `labels`, these overwrite the `rownames` of `pts`, the scores. This made the check for presence of centroid scores in biplot scores invalid as it would be very unlikely the user would supply `labels` with *exactly* the same entries as the original rownames (derived from the model.frame outputs).

This PR includes a fix which corrects this by storing the original rownames for use in the later check/comparison with the rownames of the biplot scores.