Add GetTimeFromControlPoint()

[toadative]

Use case: Two train tracks A and B meet at a point. When switching from A to B, we need to know the time of the merge point on B.

[toadative]

Given the index i of the merge point on B, the time should be B.knots[i]. If the user doesn't know the index, then we could do a linear search with fuzzy-equality.