csf_purify Function

    function csf_purify(sd_hilbert_space, total_spin, n_el) result(csfs)
        ! function to filter out all spin-allowed states from a
        ! SD Hilbert space
        integer(n_int), intent(in) :: sd_hilbert_space(:,:)
        integer, intent(in) :: total_spin, n_el

        integer(n_int), allocatable :: csfs(:,:)
        integer :: i, cnt
        integer(n_int), allocatable :: temp_csfs(:,:)

        ! we have definitely <= sds
        allocate(temp_csfs(size(sd_hilbert_space,1),size(sd_hilbert_space,2)), &
            source = 0_n_int)

        cnt = 0

        do i = 1, size(sd_hilbert_space,2)
            if (isProperCSF_flexible(sd_hilbert_space(:,i), total_spin, n_el)) then
                cnt = cnt + 1
                temp_csfs(:,cnt) = sd_hilbert_space(:,i)
            end if
        end do

        allocate(csfs(size(sd_hilbert_space,1), cnt), source = temp_csfs(:,1:cnt))

    end function csf_purify