This commit fixes a bug in the assembleResidual() function of the StructuralMechanicsModel.
In essence the commit amend 29b90b0f220.
This commit adds also a check for the lumped mass to the assembleResidual() function.
Thus it also "ensures" that that matrix exists.
NOTE: Currently this commit is a bit useless, since the model does not have the ability to assemble the matrix.
However, once it is merged with the developer branch, this should change.