Use higher level in `test_set_max_speed`

Use higher level procedures in main program
`test_set_max_speed`. Allows to read easily more information: cyclone,
`ssh_extremum`, `coord_extr` from the shapefiles. So we do not ask
this information to the user. The drawback is that we have to call
MPI (because of `read_field_indices`) even if we do not really use
it. We move the calls to `find_package` for EZMPI and `GPC_F` to the
top-level `CMakeLists.txt` since these libraries are now necessary in
both `Inst_eddies` and Overlap.