add function description to all functions in get_gap_pos.R
These functions need a few words description, parameters in and out here:
https://git.noc.ac.uk/brecinosrivas/icoads.utils/-/blob/master/R/get_gap_pos.R#L1-7
https://git.noc.ac.uk/brecinosrivas/icoads.utils/-/blob/master/R/get_gap_pos.R#L82-88
https://git.noc.ac.uk/brecinosrivas/icoads.utils/-/blob/master/R/get_gap_pos.R#L82-88
Needs to be documented also in the Wiki.
Is best if @eck can comment below and I will add it.