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).
Using atan2 is fine. I want to keep the existing behaviour, so I'll add pi to the angle to get a nonnegative angle (preserve sorting order for my tests). Do you know if std::hypot is available on gcc 4.4? I can't find a web page that gives the library status for that version.