Add a date to the OpenCV output frames - no overwriting

This came up at the last meeting.

Here is a solution:

I have a file: face.py

(jacked from https://docs.openvinotoolkit.org/latest/_docs_install_guides_installing_openvino_raspbian.html)

1. Added import datetime and import time (for good measure - I'm a hack)

2. Added variable for date_string

3. Added date_string variable to output name

4. Output the file