Use `PPR` group in Orbit files

The LISA Orbits tool provides two groups of dataset: tcb provides time series sampled according to the tcb time, while tps contains data sampled according to the tps.

At the moment, LISA Instrument uses the tcb group, and loads the ppr data set. Since the time onboard each spacecraft is supposed to be the tps, shouldn't we use the tps group?

It might also be useful to have a switch, to e.g. interface to other tools using the tt instead of the ppr, like the GW response one.

@j2b.bayle, what do you think?