[ENHANCEMENT] Rename `blockmergesort` and related functions to `sectormergesort`

Rename `blockmergesort`, `blockmergesortperm`, and `blocksortperm` to `sectormergesort`, `sectormergesortperm`, and `sectorsortperm` to make it clearer they are sorting and merging by sector.

See https://github.com/ITensor/GradedArrays.jl/pull/13#discussion_r2036125100.