Towards #278

These patches add some API points to IAtom/IBond, introduce the AtomRef/BondRef base implementations and make some small changes.

I'm still testing the Faster Implementation locally but these are some independant changes that are needed and should be relatively quick to review (compared to the imminent 'AtomContainer2' implementation).

To IAtom we add these methods:

IAtom atom = ...;
IAtomContainer container = atom.getContainer(); // the 'parent' atom container
int index = atom.getIndex(); // the index in the container
// bonds of the atom
for (IBond bond : atom.bonds()) {

}
int bondCount = atom.getBondCount();

Similar for bonds with regard to getIndex() and getContainer().

For 'current' atoms these will return null, -1, or throw unsupported operation exceptions.