add element_list

[bigd4]

It may be more convenient for pynep if add element_list back OvO

[brucefan1983]

could you also help to print out the element information here:

for (int n = 0; n < paramb.num_types; ++n) { std::cout << " type " << n << " (Z = " << zbl.atomic_numbers[n] << ").\n"; }

to amke it output something like: "(O with Z = 8)"

I think this was lost during one of my refactorization.