Support functions for constrained ordination failed cryptically with unconstrained results

Several support functions for constrained did not check if there really was a constrained component, and failed cryptically if they were applied to unconstrained ordination. They should fail because they cannot be applied, but the error message should be informative. Now it may be a message about mismatch of dimensions in QR decomposition or something equally cryptic. Traditionally we assumed that people know what they do, but this may no longer be true. I started to make these changes when I got a query of problem when the user tried to use `coef`  to extract coefficients of unconstrained ordination.