updates stimuli creation
updates stimuli creation

with this, stimuli creation now closer to final form.

 - gen_stim_set now takes a list of (w_r, w_a) tuples intead of
   generating that list itself. this is because we really don't want all
   combinations of values

 - gen_stim_set's alpha must now be a single value, not a list

 - updates main so that it now creates the 5 classes of stimuli we want:
   those that lie on the w_r axis, w_a axis, + and - 45 degree lines,
   and an arc between all those at length 32. also adds the correct
   frequencies (from 2^2.5 to 2^7.5)

billbrod/spatial-frequency-preferences,billbrod/spatial-frequency-preferences,billbrod/spatial-frequency-preferences