When we do not find an outermost contour or a max-speed contour,

instead of defining the component ssh to the ssh of the extremum or to
the ssh of the outermost contour, use a missing value flag
(1e4). Also, when radius4 >= 2 and the max speed contour is the
outermost contour, use missing value flag in the ssh component of max
speed contour instead of ssh on outermost contour. This makes clearer
output.

Add function null_ssh_contour.

In procedure get_1_outerm, simplify the comparison of level_try and
level_good. So we do not need any longer the dummy argument
ssh_extremum.