Usually C++ class instance has the same mem address as its first parent.
LuaBridge uses this to for derived classes. A TemopPoint instance has
the same address as its parent Tempo. However due to virtual inheritance
this was not the case due to a lack of virtual d'tor.
Now the following Lua code works correctly
```
tm = Temporal.TempoMap.read()
tp = Temporal.timepos_t (0)
print (tm:tempo_at(tp):note_type())
```
Previously the last line failed calling Tempo::note_type()
on a TempoPoint instance, due to memory offset e.g.
TempoPoint: 0x600000ff90e0 Tempo: 0x600000ff90e8
c8ddfd5637