DynamicAttributes refactor

This MR adds attributes (DynamicAttributes) to nodes and make them accessible from operators (as inheritedAttributes). It also includes some improvements of the DynamicAttributes class.

• Use a single mAttrs map common to C++ and Python (as suggested in eclipse/aidge/aidge_core#147 by @cguillon, however this does not really permit a clean separation with PyBind for now);
• Fix possible segfault at exit because of py::object (@mick94 this will solve your issue);
• Added a DynamicAttributes member to Node:
  • Node name is now an optional attribute (named "name");
  • Added a mechanism to pass Node attribute to the Operator (called "inheritedAttributes");
  • Attribute namespace "impl" is automatically passed to the implementation requirements (ImplSpec);
  • @cguillon I think this may be interesting for you.
• Implement hashable DynamicAttributes.

API change:

• T getAttr() const returns a copy instead of a const reference.