Added calc_angle

Set the convention for the angle of my circles with the calc_angle
function. Also changed nn_distance_angle_seperation() to use the new
function. Also added the import_matlab_gui() to accept file paths as an
argument.
Fixed bug in find_longest_traj

The max length was not being recorded in the loop for the comparison and
was only recording the last trajectory
Fixed indentation error
Changed y_axis_flip()

So it returns the new data_frame instead of overwriting the old one.
Started work on the calc_radius() function.