`PyG` is going to deprecate `TemporalData`

[M-Lampert]

I just saw that PyG is planning to integrate all time-related stuff into Data, thus, deprecating TemporalData. We need to modify our TemporalGraph accordingly. (see https://github.com/pyg-team/pytorch_geometric/issues/3230) BTW: This is also why methods like TemporalData.sort_by_time(...) or TemporalData.snapshot(...) do not work correctly. Because they are implemented for Data.sort_by_time(...) and are only inherited and the inheritance apparently does not work out of the box.