From directory Analysis. Extract corresponding tests from
`Analysis/tests.json` into new file `Inst_eddies/Analysis/tests.json`.

nx is 0 if the script is run on a triplet of shapefiles for
interpolated eddies.

This was used in old times to distinguish between triplets of
shapefiles. Now we have the philosophy of creating a dedicated
directory for a triplet. So the script `distribution_function.py` just
has to be run in the triplet directory.

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.