Working towards Jug 2.0: breaking backwards stability

[Luis Pedro Coelho]

Hello list,

I have typically been careful to never break jug backwards compatibility
(as I internally often run jug from git, that version is actually pretty
stable).

Now that the jug paper is out
(https://openresearchsoftware.metajnl.com/articles/10.5334/jors.161/), I
feel that I can have a window of breaking changes and more unstable
master.

There are a few pending github issues/PRs that were breaking that I will
now try to merge.

If you happen to be using the git version, be prepared for more
instability than in the past.

Cheers,
Luis