add method to create basis item of meso crystal

GUI/Models/MesoCrystalItem.h

@@ -52,6 +52,7 @@ public:
 
 
     ItemWithParticles* basisParticle() const;
+    template <typename T> T* createBasis();
 
     void setVectorA(const R3& vector_a);
     void setVectorB(const R3& vector_b);
@@ -70,4 +71,11 @@ template <typename T> T* MesoCrystalItem::setOuterShapeType()
     return setGroupPropertyType<T>(P_OUTER_SHAPE);
 }
 
+template <typename T> T* MesoCrystalItem::createBasis()
+{
+    if (basisParticle() != nullptr)
+        model()->removeItem(basisParticle());
+    return model()->insertItem<T>(this, -1, T_BASIS_PARTICLE);
+}
+
 #endif // BORNAGAIN_GUI_MODELS_MESOCRYSTALITEM_H