MultiTrajcetory SG interface
This MR contains modifications to MultiTrajectory to make it convenient to reuse in client codes. In essence the MTJ is split into Mutable and Const classes. The first is used while creating MTJ (e.g. in CKF) while the later lands in SG and in output files.
At the moment it is only the test that is prepared. MR is for preview by experts if this is indeed the functionality we intend. Following TDT the content will come after. @emoyse, @pagessin @adye