eliminate some compiler warnings by making copy constructors implicitly defaulted

Addresses #2222

include/assimp/color4.h

@@ -63,8 +63,7 @@ public:
     aiColor4t (TReal _r, TReal _g, TReal _b, TReal _a)
         : r(_r), g(_g), b(_b), a(_a) {}
     explicit aiColor4t (TReal _r) : r(_r), g(_r), b(_r), a(_r) {}
-    aiColor4t (const aiColor4t& o)
-        : r(o.r), g(o.g), b(o.b), a(o.a) {}
+    // aiColor4t (const aiColor4t& o) = default;
 
 public:
     // combined operators

include/assimp/vector2.h

@@ -69,7 +69,7 @@ public:
     aiVector2t () : x(), y() {}
     aiVector2t (TReal _x, TReal _y) : x(_x), y(_y) {}
     explicit aiVector2t (TReal _xyz) : x(_xyz), y(_xyz) {}
-    aiVector2t (const aiVector2t& o) : x(o.x), y(o.y) {}
+    // aiVector2t (const aiVector2t& o) = default;
 
 public:
 

include/assimp/vector3.h

@@ -69,7 +69,7 @@ public:
     aiVector3t() AI_NO_EXCEPT : x(), y(), z() {}
     aiVector3t(TReal _x, TReal _y, TReal _z) : x(_x), y(_y), z(_z) {}
     explicit aiVector3t (TReal _xyz ) : x(_xyz), y(_xyz), z(_xyz) {}
-    aiVector3t( const aiVector3t& o ) : x(o.x), y(o.y), z(o.z) {}
+    // aiVector3t( const aiVector3t& o ) = default;
 
 public: