add minDeltaTime and deltaTimeComparedTo to some get_* functions

[damianooldoni]

In get_recordt_table we can remove not independent observations by using args minDeltaTime and deltaTimeComparedTo. As discussed with @jimcasaer, it would be nice to add this feature in the following get_* functions:

• get_n_obs()
• get_n_individuals()
• get_rai()
• get_rai_individuals()

Notice that this will mean that we have to add these two args also to map_dep()! This is not the best solution, see #91! So, I think it's time to solve #91 before or while tackling this issue as well.