problems with groups of type FPGroup

[ThomasBreuer]

@fieker found these problems, see the examples shown in #1835.

The type FPGroup describes subgroups of groups that are given by finite sets of generators and relators.
On the GAP side, there is a distinction between GAP.Globals.IsFpGroup and GAP.Globals.IsSubgroupFpGroup,
and functions such as GAP.Globals.RelatorsOfFpGroup are defined only for GAP.Globals.IsFpGroup.
Thus the current Oscar function relators(G::FPGroup) does not work in general.
Similarly, forming quotients of general groups of type FPGroup may not work if the corresponding GAP group is only in GAP.Globals.IsSubgroupFpGroup; or if it works, the returned GAP object is not suitable for being used on the Oscar side, or is perhaps not reliable at all.