standardize the way we call other providers

[RMHogervorst]

F.i. in addresses and companies we call the person provider for every call.
I think it would be nicer if we put that provider in a standard slot so this is similar in all providers.

[RMHogervorst]

maybe in initialize we load those things in private with subclass(providername, self$locale)

[RMHogervorst]

The way I do it in 0.6.0 is to put the provider in a slot in private:
private$pp <- PersonProvider