predprecision simulates from the posterior predictive distribution
of the model-implied precision matrix and its determinant
using the Woodbury matrix identity and the matrix determinant lemma
predprecision(x, ahead = 1, each = 1)'fsvdraws', usually resulting from a call
to fsvsample.predloglik.Other predictors: predcond,
predcor, predcov,
predh, predloglikWB,
predloglik