Issue 34: Refactor and infrastructure for filtering functions

[athowes]

Description

This PR will close #34.

Checklist

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[athowes]

Called with @seabbs about this:

• Suggest to create new function which adds "time observed" to data
• Suggest to have function for filtering data based on time observed
• Discussion about whether "censoring" and "truncation" can happen in either order
• Discussion about whether the package should be restricted to "days" (either as date or numeric)
  • At what point along processing should dates should be transformed to numeric
• Suggest creating a mind map of functions in the package