Resolve "MultiElasticity should hold multiple MainElasticities"

Closes #213

Should be compiled and tested.

src/elasticity/solvers/CMakeLists.txt

@@ -4,6 +4,7 @@ add_library(ELASTICITY_SOLVERS STATIC
         ElasticityEuler.cpp
         ElasticityNewmark.cpp
         DynamicMechanicsSolver.cpp
+        ElasticitySolver.cpp
         )
 
 target_link_libraries(ELASTICITY_SOLVERS ELASTICITY_ASSEMBLE)
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