Add script for concatenation of triplets of shapefiles.

Generalize script `plot_snapshot.py` to read a single date in a
triplet of shapefiles containing several dates.

Bug fix in `Tests/CMakeLists.txt`: missing source file.

Include four dates in `Tests/Input/Region_1/SHP_triplet_noise`, in
preparation for a test with `test_get_snapshot`. Main unit
`test_nearby_extr` must then read the number of shapes for the first
date in `ishape_last.txt`.