Single Sampler Interface

Release Notes:

• Simplified interface to samplers.
• Added calculation of path likelihoods.
• Added support for delayed reactions.
• Added the Rejection-based SSA algorithm and Partial-propensity SSA with Composition-Rejection algorithm.
• Added support for importance sampling with mixtures.
• Plenty of new examples showing integration with Gen.jl, Turing.jl, and Survival.jl.
• Added debugging support for tracing and log messages.

The interface to individual samplers has not changed, so this is not a breaking change for most users, but the new SamplingContext interface is a major upgrade.