Write area in dbf files in km2 rather than m2. This allows easier

reading and spares some disk space: we can reduce the size of the
field from 20 to 14 characters. We do not loose precision with this
reduction because the order of magnitude of the area is 1e3 km2 or
more so the decimal part of the field was 0. Accordingly, change the
are for a null contour from - 1 to - 1e6 m2.

Bug fix in program test_set_max_speed. No need for width 20 for field "speed".