Provide a template / skeleton code for adding loaders for specific file formats

[sfmig]

Is your feature request related to a problem? Please describe.
We would like to make it very easy for contributors to add support to a variety of file formats.

Right now, it can be a bit tricky to find your way around validators and movement data structures, especially for first time contributors.

Describe the solution you'd like
Chatting with @niksirbi, he suggested providing in the docs a 'template' for:

• a load_poses.from_<generic_file> function, which takes the required file format and extracts the required numpy arrays for the dataset (see here for example).
• a Valid<generic_file> validator, that does specific checks for that file format.

Describe alternatives you've considered

• A README/text guide with steps. This may still be needed as a complement.

Additional context
Add any other context or screenshots about the feature request here.