Propagate maxh to children only in occgeom constructor

Allows resetting maxh to larger values again:

```
from netgen.occ import *
from ngsolve import *

b1 = Box((-1,-1,-1), (1,1,1))

b1.faces.Max(X).maxh = 0.1
b1.faces.Max(X).maxh = 0.2

geo = OCCGeometry(b1)
mesh = Mesh(geo.GenerateMesh(maxh=0.5))
Draw(mesh)
```

Needed for example in meshing app. Before it was not possible to set
maxh to larger value again.

libsrc/occ/occ_solid.hpp

@@ -16,6 +16,7 @@ namespace netgen
         OCCSolid(TopoDS_Shape dshape)
             : solid(TopoDS::Solid(dshape))
         { }
+        TopoDS_Solid& GetShape() { return solid; }
     };
 }
 

libsrc/occ/occgeom.cpp

@@ -1261,6 +1261,24 @@ namespace netgen
           }
       }
 
+      // Propagate maxh to children
+      for(auto& solid : solids)
+      {
+        auto& shape = static_cast<OCCSolid&>(*solid).GetShape();
+        if(!OCCGeometry::HaveProperties(shape))
+          continue;
+        for(auto& f : GetFaces(shape))
+        {
+          auto& face = GetFace(f);
+          face.properties.maxh = min2(face.properties.maxh,
+                                      GetProperties(shape).maxh);
+        }
+      }
+      for(auto& face : faces)
+        for(auto& edge : face->edges)
+          edge->properties.maxh = min2(edge->properties.maxh,
+                                       face->properties.maxh);
+
       // Add identifications
       auto add_identifications = [&](auto & shapes, auto & shape_map)
       {

libsrc/occ/python_occ_shapes.cpp

@@ -836,12 +836,7 @@ DLL_HEADER void ExportNgOCCShapes(py::module &m)
                   },
                   [](TopoDS_Shape& self, double val)
                   {
-                    for (auto typ : { TopAbs_SOLID, TopAbs_FACE,  TopAbs_EDGE, TopAbs_VERTEX })
+                    OCCGeometry::GetProperties(self).maxh = val;
-                      for (TopExp_Explorer e(self, typ); e.More(); e.Next())
-                      {
-                        auto & maxh = OCCGeometry::GetProperties(e.Current()).maxh;
-                        maxh = min2(val, maxh);
-                      }
                   }, "maximal mesh-size for shape")
     
     .def_property("hpref",