add `if metrics is None` check to compute_many (#107)
add `if metrics is None` check to compute_many (#107)

* add metrics is None check to compute_many

* use motchallenge_metrics as default in computer_overall

Co-authored-by: ezhu <1be4347fd66a1e5e9d80e77e0ac3563f95d84945@morsecorp.com>
cheind/py-motmetrics,cheind/py-motmetrics