Implement loaders for "centroid" tracking formats

[niksirbi]

Is your feature request related to a problem? Please describe.
There are several animal trackers that only track a single point per individual, e.g. idtracker and TRex, which are popular for tracking swarms of insects and fish. animovement, @roaldarbol's R package, already has loaders for such formats.

Describe the solution you'd like
Using the R implementations as a starting point, we could write similar readers for movement. The data could be probably represented as a multi-individual single-keypoint poses movement dataset.

Additional context

• See also relevant thread on zulip.
• @sfmig you will be probably interested in this, as you've been playing around with idtracker.