subgroups for Sylow

[bidundas]

It turned out that I needed a number of constructions related to subgroups for stating and proving the 2nd and 3rd Sylow theorems. Since the proofs will depend on the formulations it means that I have started to write this down even if that wasn't my plan. All of it things that obviously ought to be included somewhere, so that is some sense good.. Currently this is in section 4.11-4.13, but much of it will move elsewhere when finalized.

This means that:

1. if you need stuff like kernel, pullback, normal... please check whether it is there before starting to implement it anew
2. while the material is in flux (shaped and reshaped by how I get the best overall exposition), comments are very welcome