RichardsMechanics Initialize nonzero Stress State for Solid Models

In IntegrationPointData.h the method computeElasticTangentStiffness() calls the solid material model by solid_material.integrateStress(). At this point, the stress state is not set in the corresponding MPL::VariableArray yet. However, material behavior can depend on the stress state. This is, e.g. the case for the modified Cam clay model in its original, nonlinear elastic version. Here, the elasic moduli depend on the stress state. Zero stresses cause material integration failure.

In this MR, this bug will be fixed in such a way, that the stress state is initialized in the right moment, before the material model is called for the first time.

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