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).
> so I'll add pi to the angle to get a nonnegative angle

Well, that **might** be troublesome if you use the Boost definition because I get
```
[...]/GridUtilities.cpp:119: error: ‘boost::math::double_constants’ has not been declared
```
when targeting Boost 1.48 with GCC 4.4.  I think you're generally supposed to use
```
boost::math::constants::pi<T>()
```
rather than a specific implementation thereof.

> Do you know if `std::hypot` is available on gcc 4.4?

It doesn't matter as long as you use `std::atan2()`.