Return the cap radius as an S1Angle. (Note that the cap angle is stored
internally as an S1ChordAngle, so this method requires a trigonometric
operation and may yield a slightly different result than the value passed
to the (S2Point, S1Angle) constructor.)
Return the cap radius as an S1Angle. (Note that the cap angle is stored internally as an S1ChordAngle, so this method requires a trigonometric operation and may yield a slightly different result than the value passed to the (S2Point, S1Angle) constructor.)