Python developer tutorial should include DRE

[jbae11]

Currently, the python developer tutorial page does not include methods to implement DRE. However, the guide exists in the DRE page. Although it might be obvious to an observant developer, the Python archetype developer tutorial would be more complete if it went over the DRE implementation as well.

The C++ archetype developer tutorial implements DRE with buy_policy and sell_policy. This toolkit is not available when developing with python.

[scopatz]

I think that the C++ tutorial also has the same issue, so the tutorials should be updated simultaneously