Draft: Set body force for deformation analysis by material

This MR is for discussion only.

For excavation, shotcrete, and backfilling modeling, we need to handle the body force term of the momentum balance equation differently in relation to different modeling phases. In the excavation modeling phase, if the initial stresses are specified, the body force has to be ignored. While in the shotcrete and backfilling modeling phases, the body force should also be ignored in the rock mass, but it must be considered for the shotcrete or the backfilled materials.

This MR introduces a functionality for material-wise body force setting in deformation analysis, using the sign of the input solid density.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?