Comments have been resolved

    - New handling of measure unit for U value;
    - Hubbard matrix has been renamed everywhere as "nhubbard";
    - Function "compute_hubbard_nIJ" has been renamed as "compute_nhubbard";
    - New documentation for "compute_nhubbard" function;
    - New documentation for "Hubbard" struct;
    - New test on Wigner matrices added in tests/hubbard.jl;
    - Refactoring inside apply! for HubbardUOperator;
    - Several lines have been shortened for compactness;
    - Unutilized functions have been removed from terms/hubbard.jl;
    - Function reshape_hubbard_proj has been updated.

Author: fsicignano