Removing loop from build_dual_from_vertices

[lassejsc]

Found a trick to remove the loop sort of by accident. it does make it slightly faster.

For example for

    f = pt.vlsvfile.VlsvReader(name)
    x, y, z = np.meshgrid(
        np.linspace(0, 16, 100000), np.linspace(0, 2, 20), np.linspace(0, 2, 20)
    )
    x = dx * x * np.random.random()
    y = dx * y * np.random.random()
    z = dx * z * np.random.random()

    coor = np.vstack([x.ravel(), y.ravel(), z.ravel()]).T
    cell_vertex_sets = f.get_cell_corner_vertices(
        coorpy
    )  # these are enough to fetch the neighbours
    cell_neighbor_sets = {c: set() for c in cell_vertex_sets.keys()}
    vertices_todo = set().union(*cell_vertex_sets.values())
    
    # print(coor)
    t0 = time.time()
    arr1 = f.build_dual_from_vertices(vertices_todo)
    # print("python arr",arr)
    t1 = time.time() - t0
    f = pt.vlsvfile.VlsvReader(name)
    t0 = time.time()
    arr2 = f.build_dual_from_vertices_new(vertices_todo)
    t2 = time.time() - t0

Over ten runs gives me speed up of t1/t2 with mean t1 and t2 as

means 1.5215530983190788 old avg 0.010848474502563477 new avg 0.007144737243652344

also checked that the bboxes it produces and the output remain the same