add grass data management functions

[ninsbl]

Would be useful to have functions for listing locations and mapsets:

grassLocations()

grassMapsets(location=NULL, type) (where location can be provided or not (if not all locations are scanned) and type can be one of c("mine" (default), "general", "timeSeries", "project", "user"))

[bniebuhr]

As I understand these function are still not implemented, should I make a first implementation?
They might be useful for me as well...

[jenast]

Please do take a stab at it @bniebuhr, if you got the time.

[bniebuhr]

Hi! I will take a look at that in a few days!