Timedata

Adds properties `time_semantics` and `timestep` to `LtiSysDyn`, `PwaSysDyn`, and `HybridSysDyn` and methods to `hybrid` that checks that these values are consistent across all subsystems in `PwaSysDyn` and `HybridSysDyn`. A new example that demonstrates this functionality is in `examples/developer/pwa_timedata.py`. Existing examples should run exactly the same as before.