bug in ClassPositionsOfLowerCentralSeries

[ThomasBreuer]

Frieder Ladisch had reported the following bug:

gap> g:= SmallGroup( 3^5, 22 );;
gap> LowerCentralSeries(g);
[ C27 : C9, Group([ f3, f5 ]), Group([ f5 ]), Group([ <identity> of ... ]) ]
gap> ct:= CharacterTable(g);;
gap> ClassPositionsOfLowerCentralSeries(ct);
[ [ 1 .. 35 ], [ 1, 4, 6, 12, 15 ] ]

Looking at the code, I found related issues:

• ElementaryAbelianSeries runs into an error if the argument is a nonsolvable group.
  According to the documentation, fail should be returned.
  I think the function should be adjusted to the documentation.
  (The corresponding function ClassPositionsOfElementaryAbelianSeries returns fail
  for character tables of nonsolvable groups.)
• UpperCentralSeries returns a descending series,
  whereas ClassPositionsOfUpperCentralSeries returns an ascending series.
  The documentation of UpperCentralSeries coincides with the return value.
  Here I am not sure what to do.
• In order to run a reasonable amount of tests (on small groups) that compare the
  ClassPositionsOf... functions with the corresponding functions for groups,
  it makes sense to improve the code of some ClassPositionsOf... methods.

I will provide a pull request.

[fingolfin]

Fixed by #3347