gh-36873: add is_trivial method for groups
    
We add a method to check whether a permutation group is trivial, that
is, consists only of the identity element.

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have created tests covering the changes.
    
URL: #36873
Reported by: Martin Rubey
Reviewer(s): Dima Pasechnik, Jukka Kohonen, Martin Rubey

Author: Release Manager