Promote internal procedure get_var of procedure get_snapshot to module

procedure so it can be used by program test_set_all_outerm.

Move dummy argument corner of set_all_outerm to last position.

Bug fix in procedure get_snapshot: urc(2) must always be lower than
nlat, even if periodic is true. Fix the same bug in procedure
set_all_outerm: llc(2) >= 1 and urc(2) <= nlat even if periodic is true.

Bug fix in procedure set_all_outerm: the lower bound of dummy argument
ssh is not always 1.

Make script examine_ssh_values.py work for a periodic domain.

Make program test_set_all_outerm work for a periodic
domain. max_radius is now chosen at run-time (we must change it for
a very coarse grid). Add a corresponding test.

In written shapefiles, replace field "area" by field "r_eq_area",
that is radius of disk of equal area. This is a smaller number so it
needs fewer characters, especially for a global coarse grid.