Tracks is a bad name

After writing up #275, I am left with a strong feeling that the internal struct `Tracks` has a bad name. Why on _earth_ would a type have a name with the plural form? If I construct a `Tracks`, in my mind, I have a _single_ track, trace, or tracker. So I would much prefer `Track`, `Trace`, or `Tracker` to be the name of the type.

The way things currently are, things are not humanly readable in a meaningful way, but have to be memorized.

Any potential change would have to be made also for the `tracks` kwargument.