Add show_graphview funcionality.

Context

Adds functionality to output the description of a GraphView in a human-readable JSON format. Solves issue #160.

Added files

• aidge_core/aidge_core/show_graphview.py contains several auxiliary methods to translate the GraphView attributes to a Python dictionary and output the JSON file.
• aidge_core/aidge_core/unit_tests/test_show_graphview.py contains tests to verify that the attributes of the generated JSON correspond to the original GraphView.

@cmoineau @gregkub