Include handling of z-component and markerIDs in trajectory file

[JuleAdrian]

The import of trajectory files only handles the columns 'id', 'frame', 'x', 'y'.

I suggest including further (optional) columns:
'z', 'markerID' and maybe also 'time'

This should be handled at least in the functions <load_trajectory_data_from_txt> and in the data class .

[schroedtert]

TrajectoryData only contains data which needs to be included to start a meaningful analysis. So from my point of view we have (at least) 2 options:

1. Add the additional values to TrajectoryData. So when loading files where these are not available, a default for values like z, or marker_id need to be set. This would highly increase the memory footprint of TrajectoryData. And as we currently do not use the values, I'm not sure if this is worth it.
2. When loading TrajectoryData we drop any other column which is not in id, frame, x, and y. This could easily be changed, that all values from the file are kept. But this would also mean, that this is not a public interface of PedPy, but rather a convenience function for users.

I would highly discourage in including a time column! Rather transfer the time-values into frame-values, at least for the current state of PedPy. We maybe should discuss if for a PedPy 2.0 we should transfer the frame to a time column and remove the need of a frame rate.

[mirakuepper]

I would agree with Jule on the markerIDs. Currently I usually work around this by switching the naming of the columns 'id' and 'markerID' while reading in the data, so that PedPy drops the PetrackID and not the markerID. Considering all the effort we put into the codes on participants hats to ensure that we can correlate the trajectories to other data, dropping the markerIDs seems like a weird way to handle this...