Create `loop_shp_tr_append.py`

Replace `loop_shp_append.py` by `loop_shp_tr_append.py`. Instead of
looping on years, we assume we are working on a single year. We could
call this script inside a loop on years in the shell. The advantage is
that we need to make fewer assumptions on the arborescence of
files. Also, we can make `loop_shp_tr_append.py` more general by
starting and ending at arbitrary days. We drop the timing that was in
`loop_shp_append.py`. We add the creation of file `ishape_last.txt`.

In script `shp_tr_append.sh`, assume that the directories contain
files `ishape_last.txt` and modify `ishape_last.txt` in the
destination directory.

In program `test_overlap`, read grid description from a file
`grid_nml.txt` in SHP triplet directory, instead of from standard
input. This is the same modification we made for program
`eddy_graph`. Modify input files accordingly.