Control sections for TELEMAC-3D

Control sections are available for TELEMAC-2D. @luc.duron suggests a tentative development to do it with TELEMAC-3D, see: http://www.opentelemac.org/index.php/kunena/21-telemac-3d/13691-flux-through-control-sections-in-3d

It is based on averaged velocities along the vertical computed by TELEMAC-3D.