Fix nice monomorphism dispatch for HallSubgroup

[markuspf]

Fixes issue #557

Will this work when HallSubgroup returns a list of groups?

Also I will add some tests.

[bh11]

Could you also add the case when HallSubgroup returns a list of groups? This is the documented behaviour when there is more than one conjugacy class of Hall subgroups. SL(2,11) with π={2,3} is an example where this happens.

[markuspf]

@bh11 I just had to produce an example where HallSubgroup actually is a list of groups (I used PSL(2,7), but needed it's matrix representation, so went to the atlas...) Long story short: Yes, I am going to do this now.

[markuspf]

Voila. Also added test cases.

[hungaborhorvath]

There might be problem running this test with different packages loaded, because the presentation of the answer might change. Maybe ask for an IdGroup, or something similar?

[markuspf]

I did't realise that someone had started adjusting tests for possible isomorphisms. This is certainly not done in many places, and we do know that this is a problem, because sometimes changing algorithms gives different, but isomorphic, results.

In any case, I added IdGroup, it even works for the matrix groups, presumably because they are tiny.

[hulpke]

@markuspf Thank you very much for adding this -- when I implemented the Hall subgroup code I only tested matrix groups when using the matgrp package, and thus did not spot the need to extend nice monomorphism. Sorry!

[hungaborhorvath]

@markuspf As far As I know, two tests are run, one with all packages loaded and one with no packages loaded. Sometimes this can make differences in the output presentation. Probably @alex-konovalov could explain it better.

Anyway, thanks for this.