INTERNAL routine to apply re-parameterization from log-determinant of penalty matrix, ldetS to
model matrix, X, blockwise.
Sl.repara(rp, X, inverse = FALSE, both.sides = TRUE)reparametrization.
if X is a matrix it is assumed to be a model matrix
whereas if X is a vector it is assumed to be a parameter vector.
if TRUE an inverse re-parametrization is performed.
if inverse==TRUE and both.sides==FALSE then
the re-parametrization only applied to rhs, as appropriate for a choleski factor.
If both.sides==FALSE, X is a vector and inverse==FALSE then X is
taken as a coefficient vector (so re-parametrization is inverse of that for the model matrix).
A re-parametrized version of X.