Make the more rigorous exact kinematics limit scheme the default

This commit changes it so that if EXACT_LIMIT_SCHEME is undefined,
it has the same behavior as if EXACT_LIMIT_SCHEME!=0, instead of
EXACT_LIMIT_SCHEME==0 which was the default before. In particular,
EXACT_LIMIT_SCHEME!= sets the lower limit of the z integral to tau
and the upper limit of the xi integral to 1, instead of tauhat and
1-xahat respectively.