Add method = "cca" to weightedCor

[gavinsimpson]

weightedCor() allows for weighted correlations between WA optima from training set and an RDA or CCA axis 1 site scores of a model fitted to fossil data with WA predicted values as the sole constraint. Or at least the paper of Telford and Birks describes this. weightedCor() only allows method = "rda" and works via unexported function rdaFit(). Capability to use CCA should be added via another unexported function ccaFit().