Metropolis update for dyadic correlation
rrho_mh(Z, rho, s2 = 1,offset=0, asp=NULL)a new value of rho
n X n normal relational matrix
current value of rho
current value of s2
matrix of the same dimension as Z. It is assumed that 
Z-offset is equal to dyadic noise, so the offset should contain any 
additive and multiplicative effects (such as 
Xbeta(X,beta+ U%*%t(V) +  outer(a,b,"+")     )
use arc sine prior (TRUE) or uniform prior (FALSE)
Peter Hoff