Change some declarations IsGroup → IsSemigroup

Namely:
* DirectProductOp
* SmallGeneratingSet
* MinimalGeneratingSet

These concepts apply just as well to semigroups as to groups.
Currently, the Semigroups and/or Smallsemi packages have to
declare these separately for IsSemigroup, which then leads to
warnings when further methods are installed with the filter
IsGroup, since they match both the IsGroup and IsSemigroup
declarations.

With this change, the packages will no longer need to make such
declarations for semigroups. In the interim, I will attempt to
update Semigroups and Digraphs so that they only make their
declarations if they are not already declared for IsSemigroup,

e.g. I will check for:

[FLAGS_FILTER(IsSemigroup)] in GET_OPER_FLAGS(SmallGeneratingSet)

lib/gprd.gd

@@ -83,7 +83,7 @@
 ##  <#/GAPDoc>
 ##
 DeclareGlobalFunction( "DirectProduct" );
-DeclareOperation( "DirectProductOp", [ IsList, IsGroup ] );
+DeclareOperation( "DirectProductOp", [ IsList, IsSemigroup ] );
 
 #############################################################################
 ##

lib/grp.gd

@@ -2150,7 +2150,7 @@ DeclareAttribute( "LargestElementGroup", IsGroup );
 ##  </ManSection>
 ##  <#/GAPDoc>
 ##
-DeclareAttribute( "MinimalGeneratingSet", IsGroup );
+DeclareAttribute( "MinimalGeneratingSet", IsSemigroup );
 
 
 #############################################################################
@@ -2175,7 +2175,7 @@ DeclareAttribute( "MinimalGeneratingSet", IsGroup );
 ##  </ManSection>
 ##  <#/GAPDoc>
 ##
-DeclareAttribute( "SmallGeneratingSet", IsGroup );
+DeclareAttribute( "SmallGeneratingSet", IsSemigroup );
 
 
 #############################################################################