process: replace -m/--mets requirement by default
When running processors (core CLI or other), it should **not** be required to give `-m mets.xml` but default instead.
@@ -14,7 +14,7 @@ from ..decorators import ocrd_loglevel
# ----------------------------------------------------------------------
@click.command('process')
@ocrd_loglevel
-@click.option('-m', '--mets', help="METS to process", required=True)
+@click.option('-m', '--mets', help="METS to process", default="mets.xml")
@click.option('-g', '--page-id', help="ID(s) of the pages to process")
@click.argument('tasks', nargs=-1, required=True)
def process_cli(log_level, mets, page_id, tasks):
