Add resp. expose more combinatorics features

[fingolfin]

We already have some things (e.g. subsets from Hecke) which shoud be listed in the relevant chapter of the OScar manual.

But we also wanted to import code by @ulthiel from https://github.com/ulthiel/JuLie.jl/tree/master/src/combinatorics and possibly other sources.

[fingolfin]

Regarding subsets is also noticed that subsets(1:5, 2) doesn't work -- probably just a matter of an overly restrictive function signature

[fieker]

1:5 is no set... Can be added

[ulthiel]

@micjoswig (et. al., e.g., @ThomasBreuer) will make a start integrating some stuff from JuLie (specifically, Schur polynomials). Once there's a skeleton and I see how it works, I'll help integrating and contribute future stuff directly to OSCAR. JuLie will be archived.

[wdecker]

Sounds great!