mousing coordinates are scaled so '0 to 600 becomes -1. to 1.'
coord's undergo car-to-pol convertion
theta_value from [cartopol] converted from radian_angle (-pi to pi) to pitch_class_phase (0. to 1.)
if ( [snapPitchClass]==1 && [pv numRadii]>0 ) then
convert to degrees
add octave-level offset derived from alpha_value of [cartopol] to get theta_position on the spiral