[ITensors] Start organizing into submodules

[mtfishman]

This moves some code in ITensors into submodules. The goal is to attempt to split the library into independent parts, with an eye towards deciding what will go into the future ITensorMPS.jl package, and what else might get split off from ITensors.jl.

For example, I've moved the lattice construction code into ITensors.ITensorMPS since it is more related to the MPS code, and put the definition of the QN type into a submodule ITensors.QuantumNumbers. Additionally, I've started the process of splitting off the op, state, and val system into a submodule ITensors.SiteTypes, since it is possible that we might want to split it off from ITensors.jl and consider that code to be more of an add-on on top of ITensors.jl rather than part of the core ITensors.jl library. That code will be used by both ITensorMPS.jl and ITensorNetworks.jl, so we could think of it as sitting in between ITensors.jl and those packages.

@emstoudenmire

[emstoudenmire]

These sound like welcome changes. We can keep reducing the "surface area" between these different libraries.