Allow to use define the start value beside the stepsize in the interact ...

The interact functionality is such a wonderful tool to research your data. Thank you for getting this in.

If you specify a range between min and max you end up with the widget being set in the middle of the range.
While this certainly is a sane default behavior it is not the value you want to have in a time based plotting series, as you want to start with the initial timestep (mostly 0) instead.

This pull request allows you to specify the "value" in the interact function, so you can use it like that:

```
interact(callback, t=(0, get_last_timestep(dr2), 1, 0))
```

Happy to see some feedback