[Core] Boundary conditions and equivalent loads

[vpapado]

-Dirichlet bc -Neumann bc -Transformation of tractions and to equivalent nodal forces

[SerafeimBakalakos]

Priority list (for FEM):

1. prescribed displacements (Dirichlet)
2. prescribed tractions (Neumann): arbitrary direction and perpendicular to boundary
3. body forces
4. dynamic versions of the previous ones.
5. symmetric versions of the previous ones.