add sim2stone conversion

[erwanp]

• New Cantera-to-STONE conversion utilities : boulder/sim2stone.py module, which provides functions to convert a resolved Cantera ReactorNet into Boulder/STONE internal configuration and YAML formats (defined in add yaml config format (🪨 STONE) #7 , updated in Add new stone format, Plugins, and Custom classes #16 ).
• one Example showing a round-trip, sim => yaml => sim
• add pluggin in boulder to resolve custom mechanisms

Also add some improvements in sim2stone :

• Updated sim2stone to use the actual mechanism file from each reactor
• swap connexion direction if heat fluxes are negative
• Add CLI for sim2stone :

  sim2stone some_python_file_creating_a_ReactorNet_instance.py

And improvement in Boulder GUI :

• adjust walls/connexions colors & widths to manage overlays