Add option to use light travel times instead of proper pseudo ranges

It would be great to have (also in LISA instruments) the option to simulate the LISA measurements particularly the MPRs based on LTTs instead of PPRs. This would be very helpful for the development of the KF, it would help to study the computation of the light time corrections from the DSN-measurements step by step and not directly for MPRs simulated according to PPRs.