Fixed memory leak by adding destructors for linear equation solvers. Previously, the member unique_ptr<...> gmmxxMatrix from the gmmxx subclasses was not deleted properly

Former-commit-id: c21911b56a

src/solver/LinearEquationSolver.h

@@ -15,6 +15,11 @@ namespace storm {
         template<class Type>
         class LinearEquationSolver {
         public:
+            
+            virtual ~LinearEquationSolver() {
+                // Intentionally left empty.
+            }
+            
             /*!
              * Solves the equation system A*x = b. The matrix A is required to be square and have a unique solution.
              * The solution of the set of linear equations will be written to the vector x. Note that the matrix A has

src/solver/MinMaxLinearEquationSolver.h

@@ -19,6 +19,11 @@ namespace storm {
         template<class ValueType>
         class MinMaxLinearEquationSolver {
         public:
+            
+            virtual ~MinMaxLinearEquationSolver() {
+                // Intentionally left empty.
+            }
+            
             /*!
              * Solves the equation system x = min/max(A*x + b) given by the parameters. Note that the matrix A has
              * to be given upon construction time of the solver object.