[refactoring][api] Standardize convention for pose formatting

For now, Ceres functions takes poses input as RPYXYZ but covariance is outputed as XYZRPY

1. Goal

• Make all poses and covariances in format "XYZRPY" all along the code

2. Changes

• Actualize CeresCostFunctions.h file to switch position and orientation in pose inputs
• Actualize KeypointsRegistration startpose and endpose
• Remove covariance switch

3. Problem to merge

• The CeresCostFunctions file is used outside this project with RPYXYZ convention...

@nicolas.cadart @nick.laurenson