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