Enhance probability_contain() and remove {bpmodels} as package dependency

[joshwlambert]

This PR removes {bpmodels} as a package dependency and uses a modified version of the bpmodels::chain_sim() function to maintain the same functionality (.chain_sim()).

@sbfnk is added as a copyright holder as the author of bpmodels::chain_sim() and @jamesmbaazam is added as package contributors as the maintainer of {bpmodels}.

The functionality of probability_contain() is enhanced, partially as these enhancements are straightforward developments from the previous version of the function, and to somewhat justify the porting of the chain_sim() code. The new functionality is:

• The probability that an outbreak is contained within a given period of time can be calculated using the new outbreak_time argument. The generation_time argument is added to allow the branching process to use a generation time distribution to track the time between infections and the duration of the outbreak.

The stochastic argument has been renamed simulate to be consistent with other functions in the package that have simulate as an argument (e.g. proportion_transmission()).

The documentation of probability_contain() has been updated.

The design-principles.Rmd vignette has been updated to reflect the removal of {bpmodels}.

[github-actions]

This pull request:

• Adds 0 new dependencies (direct and indirect)
• Adds 0 new system dependencies
• Removes 1 existing dependencies (direct and indirect)
• Removes 1 existing system dependencies

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