move getContour() implementation to AbstractTriangleContourBuilder
extract nextUnvisitedCellWithContour() and genLinearInterpolateContourPoint to AbstractTriangleContourBuilder
Let NewtonSolver reconstruct solution and functionValue when start to solve()
move some private field up from MarchingTriangleContourBuilder to AbstractTriangleContourBuilder
add ContourNode
Take ContourNode instead of IntIdentityMap in TriangleContourBuilders

  * remove relative getter of IntIdentityMap of interface TriangleContourBuilder
  * let all the Node of TriangleContourCell become ContourNode after AbstractTriangleContourBuilder.prepareCellAndNodes()
  * modify accordingly other classes