Rework Accessor & Datatypes

Short description of the MR

Rework Accessor, move the virtual behaviour to the Datatypes themselves & use shared_ptr for Datatypes.

This partially solves #385

List of things to check before making a MR

Before merging your code, please check the following:

• you have added a line describing your changes to the Changelog;
• you have added unit tests for any new or improved feature;
• you have checked pdi/docs/CheckList.md
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  • you have documented with doxygen any new or changed function / class;
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