Rename FPGroups resp. clarify documentation

[fingolfin]

One thing that might confuse people: The type FPGroup can actually represent some non-finitely generated (and hence non-finitely presented) groups: there is an option to create a free group of infinite (countable) rank. And also e.g. the derived subgroup of a free group of rank >= 2 can be obtained, and is of course not finitely generated.

So in a sense, this type is for "groups given by a presentation". Perhaps it should be renamed accordingly. Then again, GAP (and I think also Magma?) have similar quirks and are apparently doing fine...