Update ofxOpenNI.cpp

so, I needed to extract the user's CoM in order to ignore any user not in the x-y-z required space.  However, I needed to make ofxOpenNI discount users that are bIsFound and bIsTracking == false even during calibration. Hence, I pulled the CoM call out of under if(g_User.GetSkeletonCap().IsTracking(nID)).... If anyone thinks I should have done it differently, let me know!
cheers,
Cindy