[IMP] purchase: Extract code from _compute_amount to allow inheritance
Purpose ======= Currently the discount mechanism is not implemented in the purchase module. This limitation is could be avoided if the _compute_amount arguments could be overwritten. While waiting the feature to be implemented in master, we simply introduce a hook method to allow an easy inheritance of the argument values. Closes #11235 PS: Also move compute methods under the fields definition to follow the coding guidelines
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