Implement loader for trackball data

[niksirbi]

Is your feature request related to a problem? Please describe.
@roaldarbol has implemented a loader for trackball data in the R library animovement. Although this is a very different data source to pose estimation, it could be in principle expressed as x,y position coordinates (as for our other data sources).

Describe the solution you'd like
Using the R implementation as a starting point, write a loader for trackball data. The data could be probably represented as a single-individual single-keypoint poses dataset.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
I'm not familiar with the file formats used in this areas, so this may require some research.
Also no-one has yet asked for this, so we can prioritise this when there is an actual need.