Merge by distance Node, must permit Vertex only input.

Created by: zeffii

https://github.com/nortikin/sverchok/blob/master/nodes/modifier_change/merge_by_distance.py

snlite script

"""
in vfields v
in doubles_distance s d=0.0 n=2
out verts v
"""
import bmesh

for vfield in vfields:
    if doubles_distance > 0.0:
        bm = bmesh_from_pydata(vfield, [], [])
        bmesh.ops.remove_doubles(bm, verts=bm.verts[:], dist=doubles_distance)

        vfield_mod, _, _ = pydata_from_bmesh(bm)
        verts.append(vfield_mod)
    else:
        verts.append(vfield)