rename getResultVertices and get_new_n_vertices

getResultVertices (get_result_vertices) is renamed to getVertices
(get_vertices) for consistency with the internal balancer interface.
Also we do not know if the returned vertices correspond to the "result"
of the balancer in this time step, since we do not know when a new time
step begins. Correspondingly, get_new_n_vertices in the Fortran
interface is renamed to get_number_of_vertices to be more clear and
consistent. (#17)