Compiling GeoModel with LTO enabled identified a One Definition Rule
warning:

```
[100%] Linking CXX shared library libGeoModelKernel.so
.../GeoModel.git/GeoModelCore/GeoModelKernel/GeoModelKernel/GeoFacet.h:24:7:
warning: type 'struct GeoFacet' violates the C++ One Definition Rule
[-Wodr]
   24 | class GeoFacet : public RCBase
      |       ^
.../GeoModel.git/GeoModelCore/GeoModelKernel/GeoModelKernel/GeoPolyhedron.h:168:7:
note: a type with different bases is defined in another translation unit
  168 | class GeoFacet
      |       ^
[100%] Built target GeoModelKernel
```

Following discussion in #77, resolve this by making `GeoFacet` in
`GeoPolyhedron.h` a protected implementation detail of `GeoPolyhedron`.
Update friendship operators and use elsewhere for nesting.