More comprehensible error message when user asks biplots of CA

There is no biplot for CA, and its results were handled by
biplot.default that gave weird error messages. Now biplot.cca() was
added to give an informative error message. An alternative to this
would be to rename biplot.rda to biplot.cca: it already has an
error message for non-rda objects, and would only handle rda.