Allow input of genetic relationship matrix for designs to optimise relationship adjacency

Probable code for genetic relatedness between columns of D (design matrix) if user passes in G.
```
rn <- rownames(G)
m1 <- match(c(D[1:(n - 1),]), rn)
mn <- match(c(D[2:n,]), rn)
rsim <- sum(G[cbind(m1, mn)]
```