Add free function vertexNeighbours() and test.

The function is needed for work related to ordered upwind methods for flow diagnostics. I decided to put it in opm/core/grid instead of making it anonymous since it may be useful to others.

The function is expected to be somewhat slow in its current incarnation, although algorithmic complexity should be ok, O(n log n).
On Mon, Dec 01, 2014 at 05:10:35AM -0800, @atgeirr wrote:
>  Do you know if std::hypot is available on gcc 4.4?

Yes it is 
```/usr/include/c++/4.4/tr1_impl/cmath-  template<typename _Tp, typename _Up>
/usr/include/c++/4.4/tr1_impl/cmath-    inline typename __gnu_cxx::__promote_2<_Tp, _Up>::__type
/usr/include/c++/4.4/tr1_impl/cmath:    hypot(_Tp __x, _Up __y)
/usr/include/c++/4.4/tr1_impl/cmath-    {
/usr/include/c++/4.4/tr1_impl/cmath-      typedef typename __gnu_cxx::__promote_2<_Tp, _Up>::__type __type;
/usr/include/c++/4.4/tr1_impl/cmath:      return hypot(__type(__x), __type(__y));```