Update trajectory file format

[ewquon]

Add header, use csv format to facilitate I/O (e.g., with pandas)

[ewquon]

May want to move away from np.loadtxt/savetxt, and use pd.read_csv/to_csv because it would handle headers better

[ewquon]

@pdoubraw do you use the find_centers(..., trajectory_file=output_text_filename) option? I'm checking to make sure this won't affect your workflow.

[pdoubraw]

never have