Minor fix in scanner example

[hailihu]

• Change in picas/clients.py: task['input']) - > task['_id']), to make it work for Tropomi tokens that do not have field input
• Move function arg_parser() from picas/util.py to local_example.py and core_scanner.py, because it is not a picas core functionality and it is confusing to set default arguments in a function that is not visible from the main program.

[lnauta]

I agree with the _id, this is always available from CouchDB.
On the parser: this is very similar code used twice. If it comes up a third time we should generalize it.
And the scanner automates picas, which should become core functionality if you ask me. Users will want to automate when job numbers become large. At that point we could add the parser too.