Making restriction estimator picklable

[danielmk]

The restriction estimator is currently not picklable, unless I am using it wrong. Since I am using it quite extensively, pickling it would be useful to me and I would be willing to look into it. Would a pull request to make the restriction estimator picklable be welcome or is someone else already working on it?

[janfb]

Yes, a PR would be welcome!

The pickling problem was also raised here: #790

Could you please create an issue describing the problem?
And then, if you like, create PR fixing it, following our contribution guidelines?
That'd be awesome! 🙏