Properties are not typesafe

When you accidentally index a property with a Handle of the wrong kind, this gives no error indication at compile time. I think VertexProperty[EdgeHandle] etc. should not compile.

PropertyPtr::operator[] currently has overloads for OpenVolumeMeshHandle and size_t arguments, IMO there should be only one function that takes the correct specific handle type. The size_t overload is dangerous in combination with the Handle's implicit int() conversion, as it would still allow this kind of type unsafety, I think we should at least not keep both, if not even remove both.

This would be a backwards-incompatible change - maybe best to handle with depreciation macros?