add parent refs and hash defs to PAGE model

[bertsky]

The Python runtime model for PAGE-XML schema generated with generateDS is not very friendly:

1. It does not provide the elements' classes with methods to access their parents (e.g. from TextEquivType to the containing GlyphType or WordType, or from GlyphType to the containing WordType asf.)
2. It does not make the elements' classes hashable and string-representable, so they cannot be used in dicts.

This makes code dealing with PAGE unnecessarily complex and less readable. I propose using generateDS' user-methods option to provide those.

[bertsky]

Also, it is very awkward that there is no general get_Regions() (irrespective of the type of region) in PageType, and no iterator for PageType / ReadingOrderType / OrderedGroupType going through all regions in order.

Ideally, the new get_Regions would have two keyword arguments:

• classes (with a list of class names) to restrict the types of regions,
• in_order (as boolean) to enumerate either by XML hierarchy or by ReadingOrder (if any).

[bertsky]

The original issue was solved by #443 and #237.

The extended feature was solved by #479 (as get_AllRegions instead of get_Regions, with regions instead of classes and reading_order instead of in_order).