Update Math.Vectors.interpolate

[DagBruck]

Performance improvements as described in #4639.

[HansOlsson]

Looks fairly good.
My only concern is that it seems as if the interface is underspecified.
Consider Vectors.interpolate({0, 2, 2}, {1,2,3}, 4)
It says that should extrapolate through the last two points, but since they have the same x-value it is unclear what "extrapolation" means.
The previous variant seems to have asserted on that (possibly by mistake due to left-over code), now we return 4.
I think the assert might be preferable - and that it should be documented.

[DagBruck]

Well, the documentation is contradictory and the check appears to be inconsistent. See example in #4639 (comment). I think you get a diagnostic when the last two x-values are the same.

We can trace this discussion back to #1555, but the alleged fix did in fact not implement what was requested.

[casella]

@DagBruck would you mind adding some test cases of the specially handled cases to ModelicaTest as well?

Thanks!