'tdt' of CWS requires a proper computation of the start time

[fpavogt]

Describe the bug
Up to now, the time array of the combined working standard were computed via the weighted average routine. In #162, a dedicated time array for the CWS has been put in place - for now, simply starting at 0 on the first step.

What is required, however, is to decide which step corresponds to the rig launch time. And for that, there is currently no routines in dvas.

@GonzagueRomanens : I'd be keen to talk to you about this at some point. Right now, I'm leaning towards using the GRUAN established launch times. These are present in the RS-41 GDPs ... to be seen if they get included in other GDPs ?

[fpavogt]

Hey @cfelix-ch, adding the cws_alt to the GDP plots requires (in part) to fix this issue.

If you and @GonzagueRomanens agree, I propose to adopt an "all-or-nothing" startegy here as well: namely, I will take the launch time to be the latest one among the GDPs. Essentially, this means that the CWS will "launch" when all three GDPs say they have launched.

[fpavogt]

After discussion with @GonzagueRomanens, we'll set the tdt=0 of the CWS as soon as 1 GDP says the flight started. This is the safest approach.