Added new background subtraction scripts

[angelinelykk]

Added 2 scripts for 2 forms of background subtraction:

1. background_subtraction_stationary.py

• Uses the first 210 frames as the background for the rest of the video
• Effective but clouds will be included. Also during the actual use case, might not be able to get the background without any smoke.

2. background_subtraction_avg.py

• Similar to the original moving average subtraction but image names are marked by time so that its easier to split them in the future instead of re-extracting every time we want to split the images in different classes