add a pca function

[rmflight]

We should just add a PCA function that does exactly what we want in terms of calculating variances, etc, and adding data back onto the sample-info data.frame if provided.

What I imagine:

• Input is a data.frame or matrix of abundances, optional data.frame of metadata, arguments to prcomp;
• Do PCA with provided arguments;
• Extract scores and loadings into data.frames;
• Calculate scores variances, and add the labels to the scores data.frame;
• Add scores to the metadata;
• Return a list containing:
  • full pca results;
  • scores data.frame;
  • loadings data.frame;
  • variance contributions data.frame;