Inconsistent behaviour between AbelianPcpGroup and AbelianGroupCons

[stertooy]

IMHO, it seems weird that AbelianPcpGroup and AbelianGroupCons share the same purpose and share a lot of code, but behave differently for certain inputs. For example, if the input contains a negative integer, we have:

gap> AbelianPcpGroup([-1,2,3]);
Pcp-group with orders [ 0, 2, 3 ]
gap> AbelianGroupCons(IsPcpGroup,[-1,2,3]);
Error, no method found! For debugging hints type ?Recovery from NoMethodFound

Since both functions share a lot of code, perhaps one could call the other? This would also prevent situations like #49.