Minor improvements to helper/checker functions

[jamesmbaazam]

This PR closes #157 by:

• Using switch() instead of ifelse() ladders for doing choice operations
• Prefixes all internal helper and checker functions with a . to align with other Epiverse-TRACE packages, e.g., superspreading, simulist, and epidemics. This is mostly for visual effects and to prevent (rare) NAMESPACE clashes.

Bullet 3 in the linked issue is solved by abstracting away the suggested change with the function update_chain_stat(), which now uses switch().

[codecov-commenter]

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