Add delay to default dataset

Created by: gowachin

Default delay of 0 is not very realist so we need to add values from real dataset regression models.

Reunion : 01/03/2023

Project : values are added to the default fit dataset and used when none is provided to make_IPM(). Maybe an additionnal pkg option to replace this with 0 when we don't want to use any delay at all (to keep tests and vignette working in a first time)

Code to produce the values is done by @kunstler, add it to data-raw for latter documentation purpose.