condense long Data::Page lists into groups

 To avoid long lists of pages, this module can condense Data::Page lists
 into a form such as:
 .
 1-6 7-12 13 14 15 16 17 18 19-24 25-30 31-36 37-41
 .
 The module supports specifying pages_per_set or max_pagesets, for
 fine-tuning.