RF: switch over to new BaseVisualStim

still to go: GratingStim and descendents
RF: switch GratingStim and RadialStim to new BaseVisuals

- all Stim classes now switched over, and all tests passing
- BUT: gruesome hack to dodge a MRO / namespace issue in RadialStim, definitely want to revise that if at all possible
- commit now anyway since everything is in a working state, revise later