outlet properties (mass flow rate, area) are not accurate for axisymmetric flows

[bigfooted]

Describe the bug A clear and concise description of what the bug is and what you expect the behavior to be instead. If applicable, add screenshots to help explain your problem. For axisymmetric flows where the outflow is aligned with the symmetry axis (horizontal axisymmetry, vertical outlet), the computed outlet area of the circular outlet can be off by 10%. This is due (I think) to the fact that the node normals are used for the computation of the edge size and integrated area and mass flux. This computation is performed e.g. in CIncEulerSolver::GetOutlet_Properties

An approach where the marker edges are used instead to compute the edge sizes leads to the correct area and mass flux computations.

To Reproduce Please provide a config file and mesh for a minimal example that recreates the problem.

Desktop (please complete the following information):

• SU2 Version: current develop