vgos, lon to longitude, lat to latitude.

Add a test for successive_overlap with periodic grid.

Add snapshot for region 4 at a different date, in input files, for a
new test of successive_overlap.

targetted a single main program.

In script plot_snapshot.py, with option --light, do not plot invalid
outermost contours.

unit test_get_snapshot. get_snapshot reads from shapefiles instead of
netCDF files.

Bug fix: programs test_get_snapshot and test_read_snapshot do not need
dispatch_snapshot.f and send_snapshot.f.

Procedures dispatch_snapshot, get_snapshot, send_snapshot and
receive_snapshot are no longer referenced, for now. They will be
called by the overlap program.

distribution_function. This allows to plot comparisons of various
combinations of runs.

Update plot_snapshot.py to work with version 2 of pyshp.

script distribution_function.py. Return dictionary instead of tuple:
easier to use.

In script plot_snapshot.py, move definition of readers to the
beginning so there is no risk of entering the window for nothing if we
forgot the end_filename option. Also, enter the four values defining
the window in a single input: easier to copy and paste.

distribution_function.py. Plot two distribution functions and two
histograms for positive speed and absolute value of negative speed.