Split tests.json into short_tests.json and long_tests.json. With ifort

and debugging options, short tests each take less than 10 s, while
long tests each take about one mn.

Bug fix in plot_snapshot.py. Slicing a multiple-dimension array with a
scalar logical does not produce an array with the same shape.

Removed computations of number of points per contour from program
test_get_snapshot. Created instead a post-processing script, stat.py,
computing these numbers of points and other stats.