Implement approx_itensornetwork (#66)

Author: LinjianMa

src/ITensorNetworks.jl

@@ -85,6 +85,7 @@ include("itensornetwork.jl")
 include("mincut.jl")
 include("contract_deltas.jl")
 include("binary_tree_partition.jl")
+include("approx_itensornetwork.jl")
 include("utility.jl")
 include("specialitensornetworks.jl")
 include("renameitensornetwork.jl")