Fail informatively if support functions for CCA are used with CA

Several cca/rda/capscale support functions only worked with
constrained ordination, but did not check their input. This resulted
in cryptic error messages such as

Error in chol2inv(Q$qr, size = rank) :
  'size' argument must be a positive integer

Now they fail with more informative error message that tells that
the method can be used only with constrained models.
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.
Merge pull request #82 from jarioksa/CCA-must-exist

Support functions for constrained ordination failed cryptically with unconstrained results