Patch to include switch for island warning as proposed in #295. The method  is modified as well to include the switch
silent_island_warning implemented for w_clip
silent_island_warning implemented for w_symmetric_difference. Previous commit included support of silent_island_warning for WSP2W as well
silent_island_warning implemented for w_difference
silent_island_warning implemented for w_intersection
silent_island_warning implemented for w_union
Merge pull request #310 from darribas/w_silent_island_warning

W silent island warning