Add tests of .getMoleculeAtoms()

[#61540920]
Implement .getMolculeAtoms() using iterative DFS instead of recursion

[#61540920]

It really speeds up models with large molecules (e.g. with proteins).
On desktop I can see that engine time is 4ms instead of 8ms, while
on iPad it's 14ms instead of 63ms! What results in FPS increase from
13 to 38.