Add marker customization to plot_trajectories

[schroedtert]

Allow independent control of start, end, and frame markers:

• traj_start/end_color: marker color (default: traj_color)
• traj_start/end_marker_size: marker size
• traj_start/end_marker_alpha: marker alpha
• traj_frame: int or list of frames to mark positions at
• traj_frame_marker/color/size/alpha: style the frame markers

Markers are now drawn with zorder=3 so they always appear on top of the trajectory lines (zorder=2).

Example call:

# color and size for start/end markers
plot_trajectories(
    traj=traj, 
    traj_start_marker="o", 
    traj_start_color="green", 
    traj_start_marker_size=10, 
    traj_start_marker_alpha=0.5, 
    traj_end_marker="x", 
    traj_end_color="blue", 
    traj_end_marker_size=50
)

# display markers at frame
plot_measurement_setup(
    traj=traj, 
    traj_frame=1000, 
    traj_frame_marker="x",
    traj_frame_color="k", 
    traj_frame_marker_size=10, 
    traj_frame_marker_alpha=0.9, 
)

To test locally:

pip install git+https://github.com/schroedtert/PedPy.git@improve-plot-trajectory-marker

Closes #536

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.07%. Comparing base (d362055) to head (93450f0).

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