core: molecule: make Molecule::clone() accepting const reference to source molecule

[mkviatkovskii]

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

Describe the solution you'd like As we are just cloning the Molecule, it should not require non-constant access to it (which could lead to unexpected side effects).

Describe alternatives you've considered Making Molecule::clone const we ensure that nothing unexpected happens to source molecule.