add elemental matrix into struct gas

[jiweiqi]

something like

E_C = [10, 0, 0, 1, 1, 3, 2, 3, 5, 6, 7, 0, 0, 0, 0];
E_H = [16, 1, 2, 3, 4, 5, 4, 6, 6, 6, 8, 0, 0, 1, 2];
E_N = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0];
E_O = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 2, 2];

This is especially useful for computing elemental conversation subspace.

[jiweiqi]

implemented at 3a5c94d

E.g.,

julia> gas.ele_matrix
6×40 Array{Float64,2}:
  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  2.0  …  0.0  1.0  0.0  0.0  1.0  1.0  0.0  0.0  1.0  0.0
 16.0  2.0  3.0  4.0  4.0  2.0  6.0  6.0  8.0  6.0  0.0     5.0  4.0  5.0  8.0  4.0  5.0  5.0  5.0  5.0  0.0
 10.0  0.0  1.0  1.0  2.0  2.0  3.0  6.0  7.0  5.0  0.0     3.0  3.0  4.0  4.0  5.0  5.0  5.0  6.0  6.0  0.0
  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0     0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  2.0    
  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0     0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0    
  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  …  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0    

julia> gas.elements
6-element Array{String,1}:
 "O"
 "H"
 "C"
 "N"
 "Ar"
 "He"