Add functions to get more series of subgroups

[fingolfin]

This is not yet done, I need to complete the docstrings. But since I mentioned lower_central_series elsewhere to @lgoettgens I thought I should at least show this here for now.

[fingolfin]

This is now ready for review. I've also added examples to a bunch of existing docstrings that had none.

[ThomasBreuer]

Nice.
(Just a few suggestions.)

As far as I understand, you want to omit the phrase "the vector of" from the description of the return value in those cases where the ordering of the entries does not have a meaning (for example in normal_subgroups), but to use "the vector of" for return values describing series.

[fingolfin]

As far as I understand, you want to omit the phrase "the vector of" from the description of the return value in those cases where the ordering of the entries does not have a meaning (for example in normal_subgroups), but to use "the vector of" for return values describing series.

Yes, that's roughly the idea. Then for e.g. subgroups in the future we maybe can return a special iterator which only computes those groups on the fly. This won't be a vector anymore.

[ThomasBreuer]

The result of the doctest says that the groups should appear in decreasing order.
I have checked with several GAP versions that this behaviour is reproducible.
How did the claimed result with increasing group orders arise?

[fingolfin]

Apparently the order changes depending on whether the CRIPS package is loaded or not.