[ITensors] [ENHANCEMENT] Investigate using `ScopedValues` for global variables

[mtfishman]

Investigate using ScopedValues for global variables, such as using_auto_fermion(), get_warn_order(), using_threaded_blocksparse(), etc.

See https://docs.julialang.org/en/v1/base/scopedvalues, https://github.com/vchuravy/ScopedValues.jl.