make `toString()` function const partons/core/partons#7

include/NumA/integration/one_dimension/IntegratorType1D.h

@@ -67,7 +67,7 @@ public:
 	 * Returns a formatted string displaying the type.
 	 * @return std::string
 	 */
-	std::string toString();
+	std::string toString() const;
 
 	/**
 	 * Returns a short name for the type.

include/NumA/linear_algebra/vector/Vector3D.h

@@ -65,7 +65,7 @@ public:
      *
      * @return std::string
      */
-    std::string toString();
+    std::string toString() const;
 
 private:
     double m_z; ///< z-coordinate.

src/NumA/integration/one_dimension/IntegratorType1D.cpp

@@ -44,7 +44,7 @@ IntegratorType1D::operator IntegratorType1D::Type() const {
 	return m_type;
 }
 
-std::string IntegratorType1D::toString() {
+std::string IntegratorType1D::toString() const {
 	switch (m_type) {
 	case TRAPEZOIDAL:
 		return "TRAPEZOIDAL";

src/NumA/linear_algebra/vector/Vector3D.cpp

@@ -29,7 +29,7 @@ void Vector3D::setZ(double z) {
     m_z = z;
 }
 
-std::string Vector3D::toString() {
+std::string Vector3D::toString() const {
 
     std::ostringstream os;