minor comments modifications
updated the model fields in the BEAM case

the model fields should always be used with the basic mesh (a 1D
interval mesh for a beam) while additional meshes can be used (here I
introduced model.virtualMesh2D) for coupling with different physics.
The coupling will have to be modified accordingly (not done yet)
added a PlotBoxAspectRatio by default in TRI6 plot
created a TODO list
relations between nodes and elements of the two meshes attached to a beam
selections of meshes depending on the two models involved in each coupling