`snapshot`. Also, move calls to `ax.gridlines` and `ax.coastlines`
inside function `snapshot`. These changes allow us to call `snapshot`
in a new figure.

Move `Region4/2006_01_01` to `Region4_2006_01_01` and
`Region4/2006_01_02` to `Region4_2006_01_02`.

Add a test of `successive_overlap` for a larger region, for which the
identifying numbers of the connected eddies are not the same for all
edges. Add corresponding input files.

Bug fix in main program `test_successive_overlap`.