Clean up check_version

[isteves]

Currently check_version turns a URL into a pid and also checks the version. We plan to organize it into two functions instead:

1. get_pid = returns a PID when given a pid/URL
2. check_version = returns the version that matches the data_pid given; returns a warning if the data_pid has been obsoleted

Need to make sure:

• integrates well with download_d1_data
• metadata matching still works