add comments

GUI/Model/Sample/LayerItem.h

@@ -49,9 +49,11 @@ public:
     ParticleLayoutItem* addLayoutItem();
     void removeLayoutItem(ParticleLayoutItem* layout);
 
+    // ambient is a top infinite or semi-infinite layer
     bool isAmbient() const { return m_is_ambient; }
     void setIsAmbient(bool b) { m_is_ambient = b; }
 
+    // substrate is a bottom semi-infinite layer
     bool isSubstrate() const { return m_is_substrate; }
     void setIsSubstrate(bool b) { m_is_substrate = b; }