Relocate IQR cleaning into egocentric_alignment() and rename function

[katiekly]

Currently, create_trainset() uses IQR cleaning to nan and interpolate over outlier points to make the pose trajectories smoother. This data cleaning step should be relocated into egocentric_alignment() and then rename egocentric_alignment() to, for example, preprocess_and_egocentric_alignment().

[luiztauffer]

it might make sense to create an explicit vame.preprocessing() step, where we can include options for:

• egocentric alignment
• normalization, IQR, filters, ...
• other preprocessing operations, such as the ones offered by movement

[luiztauffer]

covered by #119