Add new stone format, Plugins, and Custom classes

[erwanp]

Introducing Boulder Plugins :

Allows other packages so define entry points in Boulder, extending the Cantera classes with custom classes that will be imported and solved in Boulder GUI.

• BoulderPlugins container and get_plugins() to discover entry-point and env (BOULDER_PLUGINS) plugins once.
  No global registries; converters accept a plugins object.
• Converter enhancements:
  • Single converter: unchanged behavior, but coerces numeric props (MFC/Valve) to float.
    DualCanteraConverter:
    • Now uses plugin builders for custom reactors/connections.
    • Supports Wall connections; tracks walls; exposes reactor_meta.
    • Fail-fast on Cantera advance errors and non-finite states; raises RuntimeError for GUI surfacing.
• GUI robustness:
  • Simulation callback always clears “Simulating…” overlay (sets simulation-running False) and shows red error banner.
  • Error path returns a payload so client callbacks unstick even after failures.
• Housekeeping:
  Removed fragile “force-load” imports and global converter instance.
  Kept single/Dual parity for network assembly and results.

Still Todo:

• Write doc on how to set up your own pluggins

Reformatted Stone format (variation of #7 )

• Updated Boulder's internal configuration format to use "nodes" instead of "components" for better semantic consistency, while maintaining STONE format compatibility.
• Uses settings and phases instead of simulation

New 🪨 Stone format ::

metadata:
  name: "Basic Reactor Configuration"
  description: "Simple configuration with one reactor and one reservoir"
  version: "1.0"

phases:
  gas:
    mechanism: "gri30.yaml"

settings:
  time_step: 0.001  # s
  max_time: 10.0    # s
  solver: "CVODE_BDF"
  relative_tolerance: 1.0e-6
  absolute_tolerance: 1.0e-9

nodes:
- id: reactor1
  IdealGasReactor:
    temperature: 1000        # K
    pressure: 101325         # Pa
    composition: "CH4:1,O2:2,N2:7.52"

- id: res1
  Reservoir:
    temperature: 300         # K
    composition: "O2:1,N2:3.76"

connections:
- id: mfc1
  MassFlowController:
    mass_flow_rate: 0.1      # kg/s
  source: res1
  target: reactor1