AsyncGraphEdgeParams trait for easier AsyncGraph modularisation.

[daemontus]

Introduces AsyncGraphEdgeParams as a component of the AsyncGraph. Now, the responsibility of creating a coloured (parametrised) graph is split into two:

The AsyncGraph generates the state space and possible edges (possible non-deterministic updates), implementing the Graph and InvertibleGraph traits. It then calls the AsyncGraphEdgeParams which determines the colours (parametrisations) of the actual edge.

This architecture is beneficial, because it allows tweaking the graph structure without unnecessarily re-implementing the explicit state space of the non-deterministic graph. It even allows for nested (delegated) implementations of AsyncGraphEdgeParams where the DefaultEdgeParams are used as a basis for further filtering or restriction of the graph.

Note that this does not meaningfully change any existing public API of the crate (it add a reference to the signature of AsyncGraph::empty_params to match AsyncGraph::unit_params), but it adds a few new methods (aside from AsyncGraphEdgeParams and DefaultEdgeParams obviously).

[codecov[bot]]

Codecov Report

Merging #6 into master will decrease coverage by 0.83%. The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
- Coverage   85.00%   84.17%   -0.84%     
==========================================
  Files          28       29       +1     
  Lines        1761     1750      -11     
==========================================
- Hits         1497     1473      -24     
- Misses        264      277      +13     

Impacted Files	Coverage Δ
src/async_graph/mod.rs	100.00% <ø> (ø)
src/impl_variable_id.rs	0.00% <ø> (ø)
src/async_graph/impl_async_graph.rs	72.00% <66.66%> (-16.24%)	:arrow_down:
src/async_graph/impl_default_edge_params.rs	85.18% <85.18%> (ø)
src/async_graph/impl_evolution_operators.rs	91.66% <100.00%> (ø)
src/impl_fn_update.rs	87.50% <0.00%> (-6.25%)	:arrow_down:
src/impl_regulatory_graph.rs	84.90% <0.00%> (-1.89%)	:arrow_down:
src/bdd_params/impl_witness_generator.rs	81.08% <0.00%> (-1.81%)	:arrow_down:
src/sbml/mod.rs	70.14% <0.00%> (-0.69%)	:arrow_down:
src/bdd_params/impl_bdd_parameter_encoder.rs	85.45% <0.00%> (-0.61%)	:arrow_down:
... and 6 more

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