Add free function vertexNeighbours() and test.
Fix BOOST_TEST_MODULE macro.
Add orderCounterClockwise() and test.
Minor fixes for FaceCellsProxy::operator().

Fixes are:
 - Made the method const.
 - Corrected argument name cell_index -> face_index.
Use GridHelpers in implementation of vertexNeighbours().
Whitespace: tabs to spaces.
Rename vertexNeighbours() -> cellNeighboursAcrossVertices().
Replace M_PI with boost usage.
Fix comment referencing renamed function.
Fix more whitespace tab->space issues.
Simplify formula with atan2().
Use proper way to access pi() from boost::math.
Merge pull request #685 from atgeirr/vertex-neighbours

Add free function vertexNeighbours() and test.