Taking the center of the rotor circle as the origin,
get_y_grid generates a grid of vertical distances \(y\) (by
yinc increments) from the x-axis to the outer edge of the rotor
circle, across width intervals \(x\) (by xinc increments) between
the center and maximum rotor width.
Returned grid represents the left-half of the rotor's circle.
Distances are expressed as proportion of rotor radius (i.e. \(y\) is
dimensionless).