New ChBeamSection class, to host data about section Ixx Iyy and area and E G materials. this can be shared between multiple beams, that is more efficient.
Also, a ChBeamSharedAdvanced material is introduced, that allows beams with displaced centroid respect to beam reference line, and/or displaced shear center, and/or rotated section axes.
Merge branch 'develop' of https://github.com/projectchrono/chrono into develop