[ITensors] Excise ITensors.ITensorMPS

[mtfishman]

Remove the ITensors.ITensorMPS module and move it to ITensorMPS.jl.

See also ITensor/ITensorMPS.jl#31.

To-do:

• Make an announcement on the ITensor discourse page. (https://itensor.discourse.group/t/ann-itensors-jl-v0-7-breaking-release/2089)
• Clean up test dependencies. (Tracking in [ITensors] [ENHANCEMENT] Clean up test dependencies #1558.)
• Assess export list.
• Delete NDTensors.jl GPU DMRG tests (https://github.com/ITensor/ITensors.jl/tree/ITensors_excise_ITensorMPS/NDTensors/test/ITensors) and move them to ITensorMPS.jl. (It's getting removed in [NDTensors] Remove ITensors dependency from tests #1553.)
• Add note in README announcement about siteinds, OpSum, op, OpName, state, etc. being removed from the exports of ITensors.jl and instead users should load ITensorMPS.jl if they want access to them.
• Make an announcement on the ITensors.jl README/docs.
• Remove unnecessary package dependencies from Project.toml (maybe check with Aqua.jl).
• Define to_nameddimsarray(x::AbstractMPS) in ITensorMPS.jl.
• Move ITensors.ITensorMPS docs over to ITensorMPS.jl` and aggregate documentation with MultiDocumenter.jl. EDIT: I'd like to use MultiDocumenter.jl eventually but for now I just used: How to include the docstring for a function from another package in my Julia docs? JuliaDocs/Documenter.jl#1734.
• Update exports test to reflect the exports from ITensors.ITensorMPS that are being removed.
• Decide about exporting dot and inner.
• Decide about exporting ITensors.SiteTypes.

[emstoudenmire]

This will be the PR after which one must put using ITensorMPS to access that module, correct?

[mtfishman]

This will be the PR after which one must put using ITensorMPS to access that module, correct?

Yes.