Allow missing values (for continents).

For ssh, missing value is huge(0.). We have to take a large value
because Contour_531 has an argument zmax for missing values. For
velocity, missing value is a quiet NaN. We have to take this because
we need to know if the interpolation procedure in mean_speed has not
used any missing value and we do not want to alter the interpolation
procedure itself.

In local_extrema, we accept as local extrema only points with 8 valid
values around them.

When we look for the maximum speed contour, we are hoping that good
contours do not englobe missing values. If this happens, the program
aborts.

When computing the mean speed on a contour, we are hoping that the
interpolation of speed on contour points does not use missing
values. If this happens, the program aborts.

Draw coastlines in plot_snaphot.py. At the same time, this allows the
use of any projection.