Fix and enhance picking_landmarks of cedalion.plots.plot_surface()

[harmening]

This PR changes:

• Fix returned landmark labels: So far, wrong labels (always the 'Nz') were returned.
• So far, picking_landmarks was hardcoded to 'Nz', 'Iz', 'Cz', 'Lpa', 'Rpa'. Now the user can pass a list of their own/individual landmark labels to pick: set pick_landmarks to a list of needed landmark labels (instead of setting it to True). This gives the user more freedom (imagine they marked different landmarks in their photogrammetry scan) and also allows for using the function within the headmodel-workflow: If MRI segmentation is available, the user still needs to pick landmarks manually (e.g. to use the Landmarks1010-Builder). Therefore, 'Nz', 'Lpa', 'Rpa', ('Iz', can be computed from the others) are sufficient (and easier to pick).
• Improve user instructions shown during plotting:

• Return landmarks as xarray. Converting the returned values to xarray was so far done by the user (see our example notebooks 41, finger_tapping_full_pipe)
• Adapted our example notebooks that use pick_landmarks=True (12, 41, finger_tapping_full_pipe) according to the changes