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).
I'm just wondering about this particular bit.  Might it not be better to use
```C++
const double angle = std::atan2(v[1], v[0])
```
instead?  That would produce a unique angle in `[-\pi, \pi)` and you wouldn't have to fiddle with numerical approximations to `\pi` at all.  In any case
```C++
std::sqrt(v[0]*v[0] + v[1]*v[1])
```
is better spelled
```C++
std::hypot(v[0], v[1])
```
to reduce the impact of rounding errors and, if you're on a POSIX system, subnormals.