[GeoMechanicsApplication] Fix the displacement Dirichlet boundary condition

[WPK4FEM]

Currently the test for the Dirichlet boundary condition (found in test_dirichlet_u.py) is skipped. This is because the first way of fixing broke the 1D consolidation test (found in 1D_consolidation.py). This issue aims to fix the Dirichlet u test (sustainably).

Acceptance Criteria
The displacement Dirichlet boundary condition works as expected (tested by test_dirichlet_u.py)

Did a first small check with the Pw D.O.F. they react correctly to Dirichlet boundary conditions.

[WPK4FEM]

Initialization of the constitutive law in initialize solution step after computing the strain leads to a zero strain increment for Dirichlet u conditions. When the constitutive law is initialized already in initialize, a correct strain increment is found.