Use Set for GSet block systems

[mjrodgers]

When we ask for a block system for a PermGroup (using the methods blocks, maximal_blocks, minimal_block_reps, or all_blocks), the natural GSet is the set of integers from 1 to n, this is delegated to GAP and the blocks are returned as sorted vectors of integers, as a GSet with the GAP action on_sets.

When we call these methods using a GSet as an argument, we want to convert these integers back to elements of the original GSet - however in some cases, it means the vectors we have are no longer sorted and so we end up with an error when trying to define the group action on_sets. In addition, because of the generality allowed in defining a GSet, it may not even be possible to sort these vectors.

To address this, we convert the blocks returned by GAP into Julia Set objects. This does create a discrepancy in return types between the PermGroup and GSet methods, but using Set feels natural from a mathematical point of view, and allows us to effectively work withGSets in a very general way.

Fixes #4756

[mjrodgers]

We still need to document this change properly, and probably add a test that could have caught an error like this.

[ThomasBreuer]

Concerning a test, what about the example from #4756?

[mjrodgers]

Concerning a test, what about the example from #4756?

Yeah, this seems to be the smallest case where we have this issue, so I will put it in.

[fingolfin]

Looks good to me