ADD: tree_packing.py

[DamienGilliard]

This very important PR concerns tree packing with respect to the model of the structure. The first method implemented is best illustrated by this illustration:

The main changes are the creation of a packing module. Changes made elsewhere are consequences of the creation of this module, and of the later merge of the main branch to include tests

The changes require a specific test suite

[DamienGilliard]

Skeleton matching as implemented on sunday 04 august

[DamienGilliard]

!

image
As of now, the packing updates the database by removing from the database the tree part that is selected . Therefore asking to fit similar elements subsequently returns different trees, because the previously selected tree part is not in the DB anymore.
Basic but crucial

This also means that we need to "reset" the database to its original state whenever we make a mistake or want to try over.