Updated hooke.driver.picoforce to new Driver class.

Updated hooke.driver.picoforce to new Driver class.

This is a fairly major rewrite of the previous PicoForce driver.  I've
cleaned up header parsing and tried to break trace -> block conversion
up into compact functions.  The meaning of many PicoForce file
constants is still unclear to me though, so I may have gotten some
things wrong.

Also:
  * Added references for each hooke.experiment.Experiment subclass.
  * TutorialDriver sets curve.info['experiment'] to an Experiment subclass.
  * An adjusted test.curve_info now passes.

While we're translating the old experiment strings to the new
Experiment classes, the following conversion table may be useful:

  'clamp'  -> ForceClamp
  'smfs'   -> VelocityClamp
  'smfluo' -> TwoColorCoincidenceDetection