Add a script to interpose powerboards on itkpd.

This script is located in the database directory and independent of other scripts in the directory. To run it, use linux terminal and type python pbv3_add_interposer.py -p [PWB serial number(s)] or pbv3_add_interposer.py -c [panel serial number(s)]. The next step could be integrate it with the test.html so that we can use GUI to access this feature.