Create derived type `shp_triplet`. Motivation: we need to manipulate
two triplets of shapefiles in program `eddy_graph`. Use everywhere a
variable of type `shp_triplet` instead of the three variables
`hshp_extremum`, `hshp_outermost`, `hshp_max_speed` when all three of
them appear. Rename character variable `shp_triplet` to
`shp_triplet_dir`.