ns() not handled

[chjackson]

This breaks in the missing value handling code, presumably because a matrix is created for the covariate.

psor.q <- rbind(c(0,0.1,0,0),c(0,0,0.1,0),c(0,0,0,0.1),c(0,0,0,0))
psor.msm <- msm(state ~ months, subject=ptnum, data=psor, qmatrix = psor.q,  
                covariates =  ~splines::ns(months,df=3))

It would be good to support functions like ns() and bs() which create spline bases.