Measurements are no longer stored as normalized values

The value_scale has been removed from all preference_fn objects, which
ensures that measurements aren't normalized before being inserted into
the sheet_views dictionary. This also required update the testing
data to reflect the unnormalized measurement. Finally the plotting code
applies the normalization to ensure correct plotting behavior. This
commit may break backward compatibility for custom code and will require
renormalization using the cyclic_range defined in the measurement
sheet_view.
Renamed StoppedMeasurementError to MeasurementInterrupt
Renamed pattern_presenting_cmd to pattern_response_fn
Simplified CoordinatedPatternGenerator removing old code and comments

CoordinatedPatternGenerator no longer supports the retinotopy and retx
features and no longer deals with ensuring that all sheets are assigned
a pattern, which has been moved into the pattern_response command.
Fixed plotting of Hue channel by eliminating side-effects

The plotting code would previously normalize the preference values
in a sheet_view during plotting. This had the side effect that the
sheet_view itself would be divided by cyclic_range. Every time a
preference was replotted the sheet_view would be divided again,
resulting in wrong plots. New plotting code makes a copy of the
sheet_view data before applying normalization.