Bug fix: correct test for part of beginning

Correct the test to know whether we can use part of the properties
already calculated for the beginning of the segment. When `len_seg ==
2 * n_days_avg`, `g.vp.inst_eddies[n][-n_days_avg:] =
g.vp.inst_eddies[n][n_days_avg:]` so
`g.vp.inst_eddies[n][:n_days_avg]` and
`g.vp.inst_eddies[n][-n_days_avg:]` are disjoint. The bug comes from
commit bdd0ce41.