Allow `using ITensorNetworks: add!`

[mtfishman]

Since ITensor/ITensors.jl#1551 and ITensor/ITensorMPS.jl#31, ITensors.jl no longer exports add!, since it is primarily an operation for OpSum, which was also removed as an export.

As an alternative, one has to load ITensorMPS.jl which exports add!. However, we don't want to force users of ITensorNetworks.jl to explicitly load ITensorMPS.jl, so we should allow users to do using ITensorNetworks: add! to access it, and also considering exporting it eventually (I'm trying to limit exports from ITensorNetworks.jl for the time being).