Scipy-events

Changed

- Rename `core` module to `_core`, to reflect its private nature.
- Add a `factor` to `SmallDerivatives` to alter the default tolerances taken from the solver.

Added

- `Event` class to create events with terminal conditions and directions.
- `ChangeWhen` and `ChangeAt` events that continue the solution after modifying the state `y`.

Fixed

- Events `SmallDerivatives` with solver default tolerances was not working with LSODA method,
which did not expose its tolerances.

Fixed

- Events `SmallDerivatives` was not working with LSODA and BDF methods,
which did not expose the last function evaluation `f`.

Added

- `SmallDerivatives` event to solve upto a steady state.
- `Progress` event to monitor the current time with a progress bar.