In script `plot_snapshot.py`, add title with date index.

Rename `read_overlap.py` to `read_eddy_graph.py`.

Rename `loop_shpcat.py` to `loop_shp_append.py` and `shp_tr_cat.sh` to
`shp_tr_append.sh` because an analogy to the cat command is confusing:
the cat command does not modify its arguments and the result is in the
order of the arguments.

Import small NetCDF files in `Tests/Input/Region_1`.

Bug fix in `short_tests.json`: missing mpiexec for `test_read_eddy`.

New test `Eddy_graph_md2`, with corresponding input files.