Samples epistatic effects from a normal distribution or gamma distribution with a variance relative to the variance of the additive effects
sampEpiEff(qtlLoci, nTraits, corr, gamma, shape, relVar)
a matrix with dimensions qtlLoci by nTraits
total number of loci
number of traits
correlation between epistatic effects
indicator of trait should use a gamma distribution
gamma distribution shape parameter
desired variance for epistatic effects