change timespan to delay between onset and sample

[avallecam]

Calculating delays is relevant to the theme.

The calculation of the time to a fixed date could be moved to a challenge.

Instead of

tutorials-early/episodes/clean-data.Rmd

Lines 472 to 484 in 9128efa

	```{r}
	sim_ebola_data <- cleanepi::timespan(
	sim_ebola_data,
	target_column = "date_sample",
	end_date = lubridate::ymd("2025-01-03"),
	span_unit = "years",
	span_column_name = "years_since_collection",
	span_remainder_unit = "months"
	)
	sim_ebola_data %>%
	dplyr::select(case_id, date_sample, years_since_collection, remainder_months)
	```

If done, needs three updates:

• english
• french
• spanish

[Degoot-AM]

this has been addressed in #226 9b8d578