Add a bunch of trivial implications

This makes various "hidden" implications created by DeclareProperty
explicit, thus fixing 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/alg2map.gd

@@ -15,6 +15,8 @@ BindGlobal( "Family2dAlgebraMorphism",
 
 DeclareProperty( "IsPreXModAlgebraMorphism", Is2dAlgebraMorphism );
 DeclareProperty( "IsXModAlgebraMorphism", Is2dAlgebraMorphism );
+InstallTrueMethod(Is2dAlgebraMorphism, IsPreXModAlgebraMorphism);
+InstallTrueMethod(Is2dAlgebraMorphism, IsXModAlgebraMorphism);
 
 DeclareRepresentation( "Is2dAlgebraMorphismRep", 
  Is2dAlgebraMorphism and IsAttributeStoringRep,
@@ -40,6 +42,8 @@ DeclareOperation( "Cat1AlgebraMorphismByHoms",
 
 DeclareProperty( "IsPreCat1AlgebraMorphism", Is2dAlgebraMorphism );
 DeclareProperty( "IsCat1AlgebraMorphism", Is2dAlgebraMorphism );
+InstallTrueMethod(Is2dAlgebraMorphism, IsPreCat1AlgebraMorphism);
+InstallTrueMethod(Is2dAlgebraMorphism, IsCat1AlgebraMorphism);