refactor attributes'value generator to deal with yams special
Attribute/NOW/TODAY values that may be used on [Interval]BoundConstraint
(BoundConstraint wasn't handled btw, fix this).
Besides passing 'entity' to generate_* function (needed to get value for
another attribute when computing one for instance):
* set nicer method names and makes clearer what's private/protected/public
* properly handle choices, constraints on float/date/datetime fields
"""this contains the cube-specific entities' classes
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:copyright: 2001-2010 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.
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"""