Fixed bug causing incorrect pitch angle reading. Modified
PnPObj::computeEuler to return the correct angle for Theta. (Previously,
the solution for theta was chosen assuming theta could be computed as
asin(-R_3,1), but this is only one possible solution. New implementation
uses atan2 function to get phi and psi between -pi and pi, then cos(theta)
is computed and used to determine if theta is within -pi/2 to pi/2 or not.