STL-like iterator protocol for vector3

Providing iterator and const_iterator typedefs as well as begin() and
end() methods allow using vector3 with STL algorithms like `for_each` or
`find_if`.
Merge pull request #102 from Synss/vector3-iterators

STL-like iterator protocol for vector3
Merge branch 'master' of https://github.com/openbabel/openbabel into upstream
Compute vibration intensities for VASP format

The vibration intensities are not explicitely present in VASP output but
can be computed from the changes of dipole moments and the normal
coordinates.

Christian Neiss (FAU Erlangen Nürnberg) wrote the Python script that
served as the basis for this patch.