prcomp and summary.prcomp

Hi Alex, i've added only the code for prcomp and summary.prcomp. The Rd has to be renewed because here I'm using a version of roxygen2 > 4.0.0 (and one has to convert the pre 4 to 4+ doc... it's straightforward but i prefer to be you that do it if you plan to upgrade). Cheers, Luca