Change IsZ_pi from a property to a category

This seems correct, as there were no real methods to compute it. Also,
it is very similar to IsIntegers, which is also a category.

This makes a so-called "hidden" implication created by DeclareProperty
explicit, thereby resolving a bunch of warnings that show up if one
starts the upcoming GAP 4.11 with the `-N` command line option, and then
loads this package.

For some information on the background of this, see also
<gap-system/gap#1649> and
<gap-system/gap#2336>

lib/z_pi.gd

@@ -15,7 +15,7 @@
 ##
 DeclareConstructor( "Z_piCons", [ IsRing, IsList ] );
 DeclareGlobalFunction( "Z_pi" );
-DeclareProperty( "IsZ_pi", IsEuclideanRing );
+DeclareCategory( "IsZ_pi", IsEuclideanRing and IsCyclotomicCollection );
 
 #############################################################################
 ##

lib/z_pi.gi

@@ -25,7 +25,8 @@ InstallMethod( Z_piCons, "natural Z_pi (ResClasses)", true,
  IsRing and IsAttributeStoringRep ),
  rec( primes := Immutable( pi ) ) );
  SetIsTrivial( R, false );
- SetIsZ_pi( R, true );
+ SetIsWholeFamily( R, false );
+ SetFilterObj( R, IsZ_pi );
  SetZero( R, 0 ); SetOne( R, 1 );
  SetIsFinite( R, false ); SetSize( R, infinity );
  SetIsAssociative( R, true ); SetIsCommutative( R, true );
@@ -52,15 +53,6 @@ InstallGlobalFunction( Z_pi,
  return Z_piCons( IsRing, Set( pi ) );
  end );
 
-#############################################################################
-##
-#M IsZ_pi( <obj> ) . . . . . . . . . . . . . . . . . . . . . . . . . Z_(pi)
-##
-## Return false, if the contrary is not known explicitly.
-##
-InstallOtherMethod( IsZ_pi, "for non-Z_(pi) (ResClasses)", true,
- [ IsObject ], 0, ReturnFalse );
-
 #############################################################################
 ##
 #M ViewString( <R> ) . . . . . . . . . . . . . . . . . . . . . . for Z_(pi)