response enums

[janatalab]

Need to fix handling of response enums so that it mirrors the setup from Ensemble and thus handles both radiogroup and checkbox inputs.
With checkboxes, there is the possibility of selecting multiple options and this therefore requires a bitmask (power of two representation.) Along with this, we need to make sure that we have an int32 representation for response_enum so that we can support up to 32 checkbox options. We also need to add a property or method on Response that will return the index of the selected option for radiogroup questions and a list of selected options for checkbox questions. Making these will almost certainly impact existing experiment conditionals and stimulus selection scripts, so those will need to be updated also.