Make PSG Atlas-friendly

[AntonKuzin]

Added 2 new fields in the MechJebModuleStageStats. The half-stage index is determined automatically, and the end mass value is provided by the user and stored per rocket. Then the new values are passed into the SimVessel and used in the FuelFlowSimulation.

[AntonKuzin]

I'm working on an automated way to detect the 1.5 stage setup. If detected, then show the editable final mass field to the user.
You then need to find a way to optimize for the booster burnout mass. Or do it somehow else, this PR is just meant to show how I did it. If you have better idea how to calculate stages data feel free to suggest.

[AntonKuzin]

It works reliably enough now. I didn't try to deliberately fool it, so it may fail with some insane setups.
Also, the control flow is suboptimal - data flows back and forth. I'd rather put the detection logic inside the SimVessel, but it would require major staging calculation overhaul. So let's stick with this for now.

[AntonKuzin]

Hello @lamont-granquist! Do you have any comments\requests about this PR? It's been more than a month, and I really want to see it merged in the foreseeable future.

[Nazfib]

It does not work properly for me. If the end mass is set below some threshold, the stage time is completely different from when it is above that value (and the stage's end mass is also wrong). The craft file from these screenshots is attached, with the extension renamed from .craft to .txt.

Atlas LV3A Agena D.txt

[AntonKuzin]

Hello @Nazfib, it should be ok now.