SD: concentrated mass with excentricity cannot be fully accounted for in SubDyn

[ebranlard]

Currently, SubDyn users can provide inertial properties of the concentrated mass such that the concentrated mass has an offset compared to the node location, using the three lasts inputs of the concentrated mass table: MCGX, MCGY, MCGZ.

The main issue is that the FEM model of SubDyn does not keep track of the center of mass offset, and therefore, cannot compute the Guyan load correction ("extra moment") for this excentric mass. This will lead to inaccuracies in the results.

In the future, we will therefore discontinue this feature.

The workaround is:

• introduce a node at the location of the concentrated mass
• connect this node with a rigid link to the closest node of the structure

Small note: there is currently a small bug, where the gravity moment due to the excentricity of the concentrated mass is missing. It can be fixed with this change. But because in the future we will not support the excentricity any longer, there is no immediate need to fix it.

[andrew-platt]

Fixed with #2458 and included in the 4.0.0 release.