bwperim: rewrite to support N-dimensional images and connectivity.
* bwperim.m: rewrite the function in order to accept images with any number
of dimensions. Treat the connectivity argument as more standard (relying on
the new private make_conn function). Display image instead of returning its
value if nargout less than 1. Add tests.
* private/make_conn.m: new private function to be shared among functions
accepting a connectivity argument.
* NEWS: add bwperim to functions that have been changed.