Add automatic building of covariate factors

[eeholmes]

Currently

d <- rep(1:2, each=10)
MARSS(dat, model=list(d=d))

This will treat d as numeric (as it is).

But if d is character or a factor,

d <- as.factor(rep(1:2, each=10))
MARSS(dat, model=list(d=d))

MARSS will not construct the factor matrix for you.