spherical_polygon_area to spher_polygon_area.

In procedure max_speed_contour_ssh, instead of aborting when the speed
is not defined, continue with missing_ssh. Found such a case in
November 29th, 2015 data. Added corresponding test. In procedure
set_max_speed, if max_speed_contour_ssh returns missing_ssh then we
just take the outermost contour as max-speed contour.