Rationalized placement of GPIO calls:
 1. Pins are now set up when sensor objects are initialized.
 2. GPIO.setmode and GPIO.cleanup are called by main.py, within
    a try-finally block.
 3. usonic.py has a built-in test that respects this convention.
 4. The built-in test in passive_IR.py has been updated to respect
    this convention.
Fixed bug in main.py: missing import statement.