added converter to initialize std::vectors from iterable

In Python, it's now possible to do:

a=geom.Vec3(1,0,0)
b=geom.Vec3(0,1,0)
c=geom.Vec3(0,0,1)
print geom.Vec3List((a,b,c,))
print geom.Vec3List([a,b,c,])

def f(*args):
  for a in args:
    yield a

print geom.Vec3List(f(a,b,c))