use openspace fileExists function

data/assets/global/localbookmarks.asset

@@ -6,14 +6,8 @@ asset.require("scene/solarsystem/planets/earth/earth")
 
 local localBookmarks = openspace.absPath("${USER}/bookmarks/localbookmarks.csv")
 
--- Returns true if the file exists and can be opened for reading and false otherwise
-function file_exists(name)
-  local f=io.open(name,"r")
-  return f~=nil and io.close(f)
-end
-
 -- Create bookmarks file if it does not exist
-if not file_exists(localBookmarks) then
+if not openspace.fileExists(localBookmarks) then
   local file = io.open(localBookmarks, "w")
   file:write(
     "Group (optional),Name (required),Globe (optional),Lat (required if globe)," ..