Update coercion model API
|Reported by:||robertwb||Owned by:||somebody|
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Description (last modified by mabshoff)
Currently, coercions (and conversions) are defined in many different places in the SAGE codebase. We are going through each parent and consolidating/cleaning up the codebase to conform to the new coercion API, as described in the new programming guide.
This includes removing all Parent __call__ methods to use a generic __call__.