allow user to start and escape experiments with keyboard and vr controllers

[pellet]

These changes allow for some simple built-in functionality for using the oculus/quest VR controllers for kicking off experiments or exiting them early.

I've also changed the behavior of the keyboard for exiting an experiment, it now requires pressing the escape key instead of any key to exit. This helps to prevent accidental exits, but I can change it back if wanted(maybe it's going to make it harder for users to exit on purpose?).

The __clear_user_input() and __user_input() methods are used internally by the BaseExperiment class for clearing the input buffer and detecting input respectively for both keyboard and vr controllers.

The get_vr_input() and get_vr_input() methods are used internally too, but can also be used by the child Experiment classes for simple VR input, either via a xbox or touch controller button and trigger.

Any feedback on how to improve the code change will be appreciated, I've just aimed to get it working in the most basic way I could, it should hopefully give experiment writers a good starting point on how to use the controllers.