update headers to current year

This will decrease the number of pull requests from PlumedBot.

Currently, everytime a file is updated and someone forgets to run
headers.sh, the headers are outdated. With this change, this will happen
once per year. PlumedBot will thus mostly fix erroneous copyrights
(rarely happening).

src/header.sh

@@ -34,7 +34,10 @@ then
 else
     years=$(git log --follow -M75% --format=%aD $file |
     sort -r -n -k 4 |
-    awk '{if(NR==1)last=$4;}END{
+    awk -v now="$(date +%Y)" '{if(NR==1)last=$4;}END{
+# override last with now
+    last=now
+# Notice that the script could be simplified, I leave it as is for reference:
     first=$4
     if(first=="") print ""
     else if(first==last) print first;