Date-time cyclomatic complexity cleanup

[zdz2101]

Discussed in #2332

^{Originally posted by zdz2101 February 12, 2024}
We are seeking to reduce the cyclomatic complexity of the date/datetime functions in our codebase, e.g.derive_vars_dtm()

@pharmaverse/admiral please comment here if you'd like to be involved.

• Clean up easy if() and else if() code chunks in datetime/date codebase #2342
• Separate derive_date_vars() into its two inherently separate compoents: datetime vs date and their respective parent-child subfunctions #2339
• Separate test-derive_date_vars.R into its inherent separate parts  #2340
• Clean up documentation on datetime/date flagging functions, e.g. compute_dtf()/compute_tmf() to make development/maintenance easier #2343
• Add unit tests to each function and add edge cases to ensure our functions are doing what we expect (to be parsed into maybe 7-8 issues)
• Potentially collapse some of the embedded functions/wrappers so that derive_ function does not go 4-layers deep, wonder if impute_dtc_xxx() and restricted_imputed_dtc_xxx() can be more "intuitive" (tentative)

[zdz2101]

Copied description of issue into discussion #2332