Calculates Pearson's product-moment correlation coefficients
between rows in data and cluster, and is used to calculate
the position-cluster similarity matrix for the scoring function "icor".
This is implemented in Rcpp for calculation speed, using
myPearson to calculate correlations.
clusterCor_c(data, clusters)original data matrix
cluster centers
Returns a position-cluster correlation matrix as used in scoring function "icor".