Implement EJR+

[DominikPeters]

Implement a property check for the new EJR+ axiom from the paper by Markus Brill and Jannik Peters (https://arxiv.org/abs/2302.01989) which can be done in polynomial time.

(moved the function _check_JR to a more logical place in properties.py in the process)

[martinlackner]

Thanks a lot, that's a very useful contribution!