magicbot: Add marker to cause variables to be reset

[virtuald]

Magicbot programs have some boilerplate involved with variables that get reset at the end of each execute loop. An idea I have is to mark some variables to be reset to defaults at the end of each loop.

Notionally, a component using this functionality would look like this:

class Climber:
  motor = wpilib.Talon
  do_climb = magic_reset_to(False)

  def climb(self):
    self.do_climb = True

  def execute(self):
    self.motor.set(1 if self.do_climb else 0)

I imagine the implementation would be similar to how tunable is implemented: an object is stored on the class, and then at initialization time the object is switched with the default value. At the end of every loop, the defaults are placed. I imagine an efficient implementation would just store a per-class dictionary, and then do component.__dict__.update(defaults).

Haven't decided on a name for the marker yet. Some proposals were:

• magicreset
• magicdefault
• magicDefaultTo
• magic_reset_to
• will_reset_to
• magic_loop_var

Other proposals accepted!