code: def groups(self):
          
          HiisiHDF._clear_cache()
          self.CACHE['group_paths'].append('/')
          self.visititems(HiisiHDF._is_group)
          return HiisiHDF.CACHE['group_paths']
docs: Method returns a list of all goup paths
          
          Examples
          --------        
          >>> for group in h5f.groups():
                  print(group)        
          '/'
          '/dataset1'
          '/dataset1/data1'
          '/dataset1/data2'