R functions: PGLS correlation

The first function within this file calculates a phylogenetic generalized least squares (PGLS) correlation between 2 or more variables. The second function calculates the statistical significance of these correlations. The first function was derived from: Rohlf, F. J. 2006. A comment on phylogenetic correction. Evolution 60:1509-1515. See comments within the file for the appropriate use of the functions.