Do not allow loft where the segment shapes are too close ( fix #5855 )

A loft or pocket only makes sense if the segments used for it have some minimum separation. Having the shapes identical causes segfaults in OCCT

Checking for CoG is a hack. Identical shapes have identical CoG - but an identical CoG does not necessarily mean identical shapes - however it can be argued that a shape that is not offset at least somewhat in the direction of the loft doesnt make a valid shape for a loft (it will typically freeze OCCT instead of crashing it) and a sufficiently suitable shape will have a different CoG

throwing the exception will make it easy to identify if there ever is a case where this is a bad thing - and exceptions are better than segfaults.

src/Mod/Part/App/TopoShapeExpansion.cpp

@@ -4157,7 +4157,15 @@ TopoShape& TopoShape::makeElementLoft(const std::vector<TopoShape>& shapes,
                   "Need at least two vertices, edges or wires to create loft face");
     }
 
+    int i=0;
+    Base::Vector3d center1,center2;
     for (auto& sh : profiles) {
+        if (i>0) {
+            if (sh.getCenterOfGravity(center1) && profiles[i-1].getCenterOfGravity(center2) && center1.IsEqual(center2,Precision::Confusion())) {
+                FC_THROWM(Base::CADKernelError,
+                          "Segments of a Loft/Pad do not have sufficient separation");
+            }
+        }
         const auto& shape = sh.getShape();
         if (shape.ShapeType() == TopAbs_VERTEX) {
             aGenerator.AddVertex(TopoDS::Vertex(shape));
@@ -4165,6 +4173,7 @@ TopoShape& TopoShape::makeElementLoft(const std::vector<TopoShape>& shapes,
         else {
             aGenerator.AddWire(TopoDS::Wire(shape));
         }
+        i++;
     }
     // close loft by duplicating initial profile as last profile.  not perfect.
     if (isClosed == IsClosed::closed) {