data: parsing and saving inheritance

This change parses inheritance in CxxParser and saves them as EDGE_INHERITANCE in the Storage's graph. Tests for correct
inheritance recognition were added to CxxParserTestSuite.

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src/lib/data/graph/Edge.cpp

@@ -93,6 +93,8 @@ std::string Edge::getTypeString() const
 		return "has return type";
 	case EDGE_PARAMETER_OF:
 		return "has parameter of type";
+	case EDGE_INHERITANCE:
+		return "is derived from";
 	default:
 		LOG_ERROR("TypeString not implemented for edge type.");
 	}