Update JOSS paper based on reviewer two

Update the JOSS paper based on the review of @draabe.

Updated the summary to be more transparent about why blinking is important (add relevant medical papers)
Improve the statement of need to include current medical (non-image approaches) for blinking measurement
minor fixes of grammar and spelling
make the distinction between programming API and GUI more clear
give an example of how existing medical features could extend the API in the future
update reference of Lugares et al. not to use the arxiv version

cvjena / JeFaPaTo