Add methods (reserve, add) to AtomicStructureAdapter.

These methods are forwarded to the internal vector of atoms.
Remove ObjCrystMoleculeAdapter.

ObjCrystMoleculeAdapter and ObjCrystMoleculeBondGenerator were replaced
with AtomicStructureBondGenerator.
Factor small functions to AtomUtils.hpp.

Moved functions atomBareSymbol and atomValence.
Avoid inclusion of the large StructureAdapter.hpp, where they
were declared before.