rank_filter.hxx: Placed all functions in the vigra namespace.
rank_filter.hxx: Placed all functions in the vigra namespace.

nanshe-org/rank_filter,DudLab/rank_filter,nanshe-org/rank_filter,jakirkham/rank_filter,DudLab/rank_filter,jakirkham/rank_filter,jakirkham/rank_filter,nanshe-org/rank_filter,DudLab/rank_filter