Torchode

Fixed

- Keep dtype of `y` stable in mixed dtype solving when selecting the initial step size

Changed

- Clamp the timestep to the integration range even for instances that are already finished
and in the selection of the initial step size

Changed

- The time steps are now clamped so that the dynamics are never evaluated outside of the
integration range

Changed

- Make torchode compatible with python 3.8

Added

- `dt_max` option for step size controllers to set a maximum step size