Implemented SolidMechanicsModelCLIP::computeObjectiveFunction() (to be tested)

This function will be necessary to compute d by solving optimisation problem. Main developpement:

• Added MaterialBulkDamage::computeObjectiveFunction() and MaterialCohesiveDamageExtrinsic::MaterialCohesiveDamageExtrinsic + necessary subfunctions
• Now G_c and sigmac_c have to be parsed by MaterialBulkDamage (it needs it for Yc and lch)

Minor fixes:

• Fixed usage of some parameters which where public instead of protected + added accessors
• Fixed material.dat file in clip_compute_damage example
• Removed warning in MaterialCohesiveDamageExtrinsic which was allowing its use only with cohesive extrinsinc model