Code cleaning: replace import() by importFrom()

[TomSnijders]

Replace import(Matrix, lattice, parallel, MASS) in the NAMESPACE by importFrom() for the functions that are actually used. This will make the R code more explicit and reduce package size (I expect).

[jhollway]

I'm not sure about package size, but it should help avoid NAMESPACE conflicts.

[TomSnijders]

I actually thought of this because of a discussion on [email protected] that was about a NAMESPACE conflict. That it would lead to a smaller package size is just a guess and may be mistaken.