Convert duplicated code into helper functions

[jamesmbaazam]

• Please check if the PR fulfils these requirements

• [x] I have read the CONTRIBUTING guidelines

• [x] The commit message follows our guidelines

• [x] Tests for the changes have been added (for bug fixes / features)

• [x] Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• It refactors simulate_chains(), simulate_summary(), and likelihood() to remove duplicated code by converting them into internal helper functions follows:

  • Simulation related operations have been modularised into .init_susc_pop(), .sample_possible_offspring(), and .get_susceptible_offspring().
  • Input checking is now remitted to 3 functions, .check_sim_args(), which checks the shared arguments of the simulate_* () functions, and 2 smaller functions, .check_statistic_args() which checks stat_max and statistic together in multiple functions, and .check_time_args(), which checks the time-related arguments: generation_time, t0, and tf.

• Tests and documentation, where necessary, have been added.

• What is the current behaviour? (You can also link to an open issue here)

This is a refactoring exercise and leads to no user-facing behaviour change.

• What is the new behaviour (if this is a feature change)?

  This is a refactoring exercise and leads to no user-facing behaviour change.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No.

• Other information:

• This PR closes #44.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d22fd51) 98.90% compared to head (56c0a74) 98.94%.

:exclamation: Current head 56c0a74 differs from pull request most recent head 32ccc24. Consider uploading reports for the commit 32ccc24 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #199 +/- ## ========================================== + Coverage 98.90% 98.94% +0.04% ========================================== Files 8 8 Lines 550 571 +21 ========================================== + Hits 544 565 +21 Misses 6 6 ```

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