Add .Page.Group function to create a PageGroup object

The implementation use a method as using a plain function (in tpl) leads to circular imports
The function would need the PageGroup object, hence the Page object.
Both are in hugolib/* that import tpl that would need to import hugolib/Page

User can now do
  {{ $cool := .Page.Group "cool" (where .Site.RegularPages "Params.cool" true) }}
  {{ $blue := .Page.Group "blue" (where .Site.RegularPages "Params.blue" true) }}

hugolib/pageGroup.go

@@ -296,3 +296,10 @@ func (p Pages) GroupByParamDate(key string, format string, order ...string) (Pag
 	}
 	return p.groupByDateField(sorter, formatter, order...)
 }
+
+// Group creates a PageGroup from a key and a Pages object
+func (p *Page) Group(key interface{}, pages interface{}) (PageGroup, error) {
+        pageGroup := PageGroup{Key: key, Pages: pages.(Pages)}
+
+        return pageGroup, nil
+}