Create Worldlines from different kinds of descriptions of a worldline

[kurtamohler]

Currently, Worldines are created from a list of vertices (plus optional past and future velocities), but a list of vertices is just one way to describe a worldline. Sometimes other kinds of descriptions are more convenient, and it can be cumbersome for the user to have to calculate the vertices explicitly from these descriptions.

For instance, in the twin paradox, the worldline of the moving twin in the rest frame can be described by:

• initial event in spacetime
• velocity during first leg of journey and the distance traveled during first leg
• velocity during second leg and the distance traveled during second leg

For an arbitrary number of legs of a journey described in this way, we would have a list of velocity-distance pairs for each leg.

Another slightly different way to describe the worldline in the twin paradox is:

• initial event in spacetime
• speed during first leg of journey and the position in space of the end of first leg
• speed during second leg and the position in space of the end of the second leg (which is just the initial position in this case)

So there should a way to generate a Worldline from a handful of different common ways to describe a worldline.

One could potentially mix multiple different ways of describing the legs of a journey. For instance:

• initial event in spacetime
• speed during first leg and the destination position in space
• velocity during second leg and the distance traveled
• the event at the end of a third leg

Supporting this mixing of types of descriptions would probably be useful.

Also, if I can design a fairly simple and easy to use API for it, this may be a good replacement for the awkward vel_ends argument of torch.Worldline.__init__