Use identifier in the person class instead of a "single-attribute class" student / learner

[init-xbildung]

Learner and Student class functionality currently is limited to hold a "learnerID" or "studentID" property.
Meanwhile the person class has also an identifier.

We recommend to use the identifier of the person class and to get rid of the redundant layer "learner" and "student"

+++ This issues was raised by HIS eg and TU Braunschweig
within the 2022 XBildung Call for Comment on SDG Data models. +++

[EmielPwC]

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