values at longitude boundaries, in case of periodicity.

Add argument "periodic" to procedures get_snapshot and
set_all_outerm. Duplicate values at longitude boundaries in ssh, u, v
if periodic. set_all_outerm can now call local_extrema with periodic
true. In procedure set_all_outerm, we shift the longitudes of outside
points if necessary.

In program test_get_snapshot, define periodic from step and nlon.