Type "help polyclass" on the new megagenetic analysis function. The
pedigree needs to be recoded to have a class field to identify
separate populations. Covariance regression and inverse gaussian
normalization is performed by "class". Mega-genetic heritability is
calculated together with hertiability for different classes.
See http://www.ncbi.nlm.nih.gov/pubmed/23629049 for more