FSI3D: relative system

[tommy-engels]

in the FSI3D module, we set up the beam reference frame in the same spirit as we do for the insect body, even with the same notation.
However, the BEQ solver was originally for 2D codes, where we didn't implement such a functionality. The oddity is now that in
subroutine mouvement
we set the leading edge coordinate to zero, even for a heaving motion. the same for the velocity. only the acceleration is non-zero, since it enters the BC for the BEQ
both are set in
subroutine plate_coordinate_system
this is however confusing and should be altered