Various fixes to make unittests pass
Cosmetic
* Raise exceptions on calls after stop()
* Added clear_table method
PEP8 cosmetics
* Making sure we can call init() twice
* Clear all values in data_table instead of droping the table
Adding alias for end(): restart(). Should get more code in a while
Keep track of first scan, restart things that need to be restarted before starting the N'th scan (N>1)