Rename file `test_write_eddy.f90` to `test_write_null.f90`. The name
was badly chosen because the program is not a unit test of procedure
`write_eddy`. In fact, it does not even call `write_eddy`.

Changing my mind. In an SHPC, store shapefiles in
"Slice_{slice}/{orientation}" instead of
"{orientation}/Slice_{slice}". Two reasons: easier to store accessory
files from running `inst_eddies`, such as `perf_report` and output
from Torque submission script; clearer that a slice should contain the
same dates for both orientations.

Assume that a slice is in a directory
"{shpc_dir}/{orientation}/Slice_{slice}" where shpc_dir can be any
name, orientation is "Anticyclones" or "Cyclones" and slice is an
integer. Do not read or create any longer a file "orientation.txt" in
the slice directory. Add procedure `get_slice_dir`. Add dummy
arguments cyclone and slice to procedure `shpc_open`. Add dummy
argument slice to procedure `shpc_create`.

Because it contains timing.