upgraded to Armadillo 4.399.2 pre-release
update DESCRIPTION and NEWS.Rd
cherry-picked 'possible fix for sparse matrix conversion'
cherry-picked 'further correction to sparse matrix converter'

special thanks to Ryan Curtin; this now sets sentinels for all three vectors
as preferred internally by Armadillo
Merge pull request #18 from RcppCore/feature/arma-4.399.2

Feature/arma 4.399.2