User friendly way to select reference points for alignment

[luiztauffer]

Currently, we define which body parts should be used as reference like this:

vame.egocentric_alignment(
    config,
    pose_ref_index=[7,8],  # aligning on pelvis and tail-base
)

This is not user friendly and prone to errors. A more user friendly solution would be accept a list of body parts names, from which we can infer the indexes, example:

vame.egocentric_alignment(
    config,
    pose_ref=["pelvis", "tail-base"], 
)

The vame function would search for the correct column indexes, and raise an error if the names don't exist.