Calculation of XYZ

[AndreyTavleev]

In MESA rep in kap_lib.f90:kap_get() there is useful method of X calculation:

 xc = 0; xn = 0; xo = 0; xne = 0
 do i=1, species
    iz = chem_isos% Z(chem_id(i))
    select case(iz)
    case (6)
       xc = xc + xa(i)
    case (7)
       xn = xn + xa(i)
    case (8)
       xo = xo + xa(i)
    case (10)
       xne = xne + xa(i)
    end select
 end do

I think we should use it too.