#185 Additional util methods

[John May]

The 'isEmpty' feature request got me thinking of some other useful utilities that would be simple to implement and in my opinion improve usability and readability.

- `.clear()` remove all atoms/bonds from the AtomContainer/ContainerSets etc. - simply invokes the `removesAllAtomsAndBonds`
- `addAtom()` and `addBond` could return true/false as to whether the container was modified - this is useful as there is currently no indication when duplicates are added.

~~~~~:::java
public void addAtom(IAtom atom)
{
if (contains(atom))
{
return; // false
}

    if (atomCount + 1 >= atoms.length)
    {
        growAtomArray();
    }
    atoms[atomCount] = atom;
    atomCount++;
            // return true
}

~~~~~

[John May]

Oh and we can also do this...

    public void add(IAtomContainer atomContainer);

    public void add(IAtom atom);

    public void add(IBond bond);

currently code reads as follows:

container.addAtom(atom) - container add atom atom

this could easily be reduced to

container.add(atom) - container add atom

The existing methods could stay but the util methods would make implementing code look a lot cleaner.