Build DAG then create an ordered list of gates from it. DAG starting
points could be Net or Instances (we take the output of the Instance).
Multiple DAG can be created, but once an Instance is part of a DAG,
it connot be part of another and is conidered as "reached" by all
subsequent DAG run. The ordered list build from a DAG contains both
Net and Instances.
   A new property has DagProperty, accessible through DagExtension
has been created to store relevant information of the DAG on each
Instance or Net (work in progress).

* New: Isobar::PyAttributesHolder, a PyObject with only a dictionary
    to hold the attributes associated to a DBo.
* New: Isobar::PyHolderProperty, the Property that encapsulate
    PyAttributesholder.
* New: Isobar::PythonAttributes, the extension to simplify the
    management of the PyAttributesholder.
* Change: In PyEntity, now use a dedicated tp_getattro and tp_setattro
    to delegate the Python attribute access towars the PyAttributesholder.
* New: In hurricane/doc & documentation, update docs regarding Python
    attributes managment.

This reverts commit 0e955667.

This reverts commit 1016ae54.

This reverts commit 0e955667.

Conflicts:
	Seabreeze/src/Delay.cpp

Main list of ports:

* Replace deprecated operator '<>' by '!='.
* To check if a list is empty, do not compare to [], but check it's
  length instead.
* Do not make a class inherit indirectly twice from the same base class.
* Hurricane physical object constructors uses DbU::Unit as arguments,
  seen as int in Python, so they will not match float. Unfortunately
  the calculation often gives float. So explicitely cast them into
  int. This is due to a change of behavior in Python. Now, 3/2
  gives 1.5 (float). To get the previous one use: 3/2 -> 1 (int).
* dict.keys()[0] no longer work, instead use list(dict.keys())[0].

In SelectorCriterion & SelectorCriterions, when selecting a Net occurrence,
we where storing the Net only. This was fine if the Net was belonging to
the Cell's top level. But when it was an occurrence of a non-top level
net, this was creating the elusive incoherent Occurrence problem.
Now we truly store the occurrence of the Net, to be accurate, the root
of the HyperNet.

* Change: In SelectorCriterions::add(), the Net* argument is replaced
    by a Occurrence of Net. Must be an HyperNet root occurrence.
      Same goes for SelectorCriterions::remove().
* Change: In CellWidget::select(), when called with a Net occurrence,
    select the whole HyperNet components.
* Change: In NetSelectorCriterion, now use a Net occurrence instead
    of directly a Net. Must be an HyperNet root net occurrence.