Flat viewing model for Signals

[k1o0]

Is your feature request related to a problem? Please describe. The Signals viewing model is pseudocircular, meaning the stimuli are warped.

Describe the solution you'd like There should be an option to have a single screen 'flat' viewing model that does the same thing as |hw.BasicScreenViewingModel|. This should be set by the 'screens' rig object. Ideally the screens field should be a hardware viewing model (a subclass of hw.ViewingModel) so that it can be properly parameterized.