Added fold

[Ved-Mahajan]

Added the function fold that returns a VectorFold which when unfolded gives the original vector.
Example:

v1 = VectorFold([42,3,45,6],13,4)
w1 = unfold(v1)
v11 = fold(w1)

julia> unfold(v11) == w1
true

v2 = VectorFold([34,34,43,43],10,3)
w2 = unfold(v3)
v22 = fold(w2)

julia> unfold(v22) == w2
true