Add support for namespaced attributes in DynamicAttributes

Add support for namespaced attributes in DynamicAttribute with a recursive approach (a DynamicAttribute can have an attribute of type DynamicAttribute). Usage example (both syntaxes are correct):

attrs.get_attr("mem.data.b")
attrs.get_attr("mem").get_attr("data").get_attr("b")

This is a preliminary work in order to introduce a general mechanism for specifiyng backend and export attributes.

• Needs more tests in Python.
• Use snake convention for all attributes naming in C++ and Python.
• Solve #142 (closed)