adaptation to recent changes in carl

Former-commit-id: 90535a0d66

src/storage/expressions/ToRationalFunctionVisitor.cpp

@@ -65,7 +65,7 @@ namespace storm {
             if (variablePair != variableToVariableMap.end()) {
                 return convertVariableToPolynomial(variablePair->second);
             } else {
-                carl::Variable carlVariable = carl::VariablePool::getInstance().getFreshVariable(expression.getVariableName());
+                carl::Variable carlVariable = carl::freshVariable(expression.getVariableName(), carl::VariableType::VT_REAL);
                 variableToVariableMap.emplace(expression.getVariable(), carlVariable);
                 return convertVariableToPolynomial(carlVariable);
             }