First commit (move code from ITensors.jl)

[mtfishman]

This is essentially the same code as ITensors.SiteTypes, but with a few tweaks to make it compatible with ITensorBase.jl.

[emstoudenmire]

I agree it's a good time to split these off.

[mtfishman]

I agree it's a good time to split these off.

@emstoudenmire glad you agree. I also plan to experiment with some new designs here, and delete code that we've been keeping around for backwards compatibility that is making things unnecessarily complicated and hard to modify/maintain.

Note that I chose a deliberately verbose name since I also plan to split this up even more, for example I want to split off parts that aren't specific to ITensor into a package QuantumOperatorDefinitions.jl, and then this package can probably just be a package extension that gets triggered to load when ITensors.jl and QuantumOperatorDefinitions.jl are both loaded. (EDIT: And of course I'm open to other names, nothing is set in stone yet.)