Remove uses of arma::schur, that have a different semantics in Armadillo 6.100

[jhjourdan]

[armstrtw]

I've moved the loglik functions here: https://github.com/armaMCMC/arma-log-likelihood

and shur_prod has been updated in that repo. if you still think the semantics need to be chagned (std::forward, etc), then please open a PR there.