refactored 4-index op build
added DES op and refactored 4-index disk build
refactored 4-index disk build
refactored 3-index build to be analogous to 4-index build; refactored renormalize_transform_on_disk
refactored 3pdm parallel build to remove duplications
refactored 4pdm parallel build to avoid duplications
optimization: reduced cost of generating all spin-permutations; removed 100% of redundant spin-orbital elements
optimization: reduced cost of spin-adapted transformation