Rewrite of fortify_pca

Use sub-functions with methods to extract eigenvalues, scores and data. This allows to have just one function called for all methods.
Add scaling based on the conventions in vegan.
Make sure cos2 and contrib still follow the conventions of FactoMineR.
Use roxygen better for the documentation.

Author: jiho

NAMESPACE

@@ -2,13 +2,20 @@ S3method(autoplot,PCA)
 S3method(autoplot,pcaRes)
 S3method(autoplot,prcomp)
 S3method(autoplot,rda)
+S3method(eigenvalues,PCA)
+S3method(eigenvalues,pca)
+S3method(eigenvalues,pcaRes)
+S3method(eigenvalues,prcomp)
+S3method(eigenvalues,rda)
 S3method(fortify,PCA)
+S3method(fortify,pca)
 S3method(fortify,pcaRes)
 S3method(fortify,prcomp)
 S3method(fortify,rda)
 S3method(plot,ggplot_list)
 S3method(print,ggplot_list)
 export(autoplot_pca)
+export(eigenvalues)
 importFrom(ggplot2,autoplot)
 importFrom(ggplot2,fortify)
 importFrom(grid,arrow)