add special objects for subgroup transversals

[ThomasBreuer]

Use GAP's IsRightTransversalRep objects instead of "unpacking" the transversal in Oscar.

Currently the PositionCanonical functionality from GAP is not available for the left/right transversals.
This means that Oscar does not support to "act on the transversal" in the sense of GAP.

resolves #3197

[fingolfin]

OK great!

Now the next thing on my wish list is that right_cosets returns a G-set, so that I can rewrite the following example from the book:

G = dihedral_group(6)
U = sub(G, [g for g in gens(G) if order(g) == 2])[1]
r = right_cosets(G, U)
perm([findfirst(isequal(c*G[1]), r) for c in r])  # get action of G[1] on the cosets in r

to instead use this in the last line:

permutation(r, G[1])

I can kind of fudge it right now but only by using the undocumented Oscar.GSetByElements directly:

x = Oscar.GSetByElements(G, *, r; closed=true)
permutation(x, G[1])

[ThomasBreuer]

@fingolfin
Currently you could also use the documented gset(G, *, r, closed = true), and then call permutation or action_homomorphism.

But this is of course not what one wants, the idea is to create a new G-set type that stores a right_transversal object instead of the explicit vector of coset objects, and to change right_cosets to return such a G-set.
(Then the return value of right_cosets will no longer be a vector. Currently right_cosets seems to be not used anywhere, this change is perhaps acceptable.)