Added vignettes, readme.
Initial CRAN submission.
Renamed the R implementation of scalable PCA to pca_spca_R() and added a
C++-backed pca_spca() along with pca_spca_stream_bigmatrix() for
bigmemory::big.matrix workflows, retaining the block power iteration
algorithm of Elgamal et al. (2015).
Added pca_spca() implementing the scalable PCA algorithm of Elgamal et
al. (2015) with streaming block power iterations for large matrices.
Added an iteratively reweighted singular value decomposition backend for
pca_robust() that exposes the final row weights and iteration count.
Implemented a compiled svd_robust() helper with an svd_robust_R()
reference implementation.