json parser: change + to @ for URLs (#175)

Since `+` is used in algebraic expressions, this char is unsuitable
for the indication of external URL resourcs as used by the importer of
actinia (https://github.com/mundialis/actinia_core/).

Fixes https://trac.osgeo.org/grass/ticket/3928

lib/gis/parser_json.c

@@ -344,7 +344,7 @@ void check_create_import_opts(struct Option *opt, char *element, FILE *fp)
     int has_import = 0;
     char **tokens;
 
-    tokens = G_tokenize(opt->answer, "+");
+    tokens = G_tokenize(opt->answer, "@");
     while (tokens[i]) {
         G_chop(tokens[i]);
         i++;