Make rand_pseudo(::GAPGroup) uniformly usable

[fingolfin]

Now one can always specify radius and it has a default value

This does not address the deeper issues mentioned in #1165 but
at least it makes it a bit more usable for the moment.

[ThomasBreuer]

What about describing the behaviour explicitly in the documentation:

    rand_pseudo(G::GAPGroup; radius::Int = 10)

Return a pseudo random element of `G`.

If `G` is an `FPGroup` then the result is a word in the generators of `G`
in the ball of radius `radius`, with equal distribution in the underlying free group.

If `G` is a finite permutation or matrix group,
`rand_pseudo` uses a variant of the product replacement algorithm.
For most inputs, the resulting stream of elements relatively quickly converges
to a uniform distribution.

The idea is that `rand_pseudo` returns elements that are cheap to compute
(cheaper than with `rand`) and somehow random,
but makes no guarantees about their uniform distribution.

It is sometimes necessary to work with finite groups that we cannot
effectively enumerate, e.g. matrix groups over finite fields. We may not even
know the size of these groups. Yet many algorithms need to sample elements
from the group "as randomly as possible", whatever that means; but also they
need this *fast*.

[fieker]

I would not worry with the exact description at this point, as the random interface, the idomatic julia one at least, is different anyway (see underlying issue as well). Thus I will merge to get it over and done with