[NDTensors][ITensors] Refactor BlockSparse system

[kmp5VT]

Description

The goal of this PR is to refactor the BlockSparse system. A few things will go into this refactor: 1. I will be organizing the code to match the previous NDTensors refactors, 2. I will be working to make a more understandable and documented blocking system in ITensors, 3. I will be working to make the BlockSparse system stand-alone from the ITensors package, 4. I will be updating the blockoffset system to utilize either Tensors or StorageTypes which will allow more flexibility in the sparsity of these Tensors.

This PR is based on #1097

Checklist:

• My code follows the style guidelines of this project. Please run using JuliaFormatter; format(".") in the base directory of the repository (~/.julia/dev/ITensors) to format your code according to our style guidelines.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that verify the behavior of the changes I made.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.

[mtfishman]

@kmp5VT can this be closed? We will be moving to the BlockSparseArray type anyway.