Using viscose methods in assemble when needed

conf/elasticity/elasticity.conf

src/elasticity/MainElasticity.cpp

src/elasticity/MainElasticity.hpp

src/elasticity/assemble/IElasticity.hpp

src/elasticity/solvers/DynamicMechanicsSolver.cpp

src/elasticity/solvers/DynamicMechanicsSolver.hpp

src/elasticity/solvers/ElasticityEuler.cpp

src/elasticity/solvers/ElasticityEuler.hpp

src/elasticity/solvers/ElasticityNewmark.cpp

src/elasticity/solvers/ElasticityNewmark.hpp

test/elasticity/CMakeLists.txt

test/elasticity/TestViscoElasticity.cpp