[WIP] adding output at specified points

I just added the `initstep`, `minstep` and `maxstep` keywords specified in the API.
Output at specified time-points is still wip, as for a `tspan` of `[0.:10.:50.]` I currently get output at

```
tout = [0.0 6.0e-7 2.351e-5 0.0005 0.009 10.013 19.981 30.016 39.983 49.997 50.0]
```