new_contents: #!/usr/bin/env python3
  from __future__ import print_function, division
  import argparse
  
  parser = argparse.ArgumentParser()
  parser.add_argument('file', type=str, help='H5MD datafile')
  parser.add_argument('--plot', action='store_true')
  args = parser.parse_args()
  
  import numpy as np
  import h5py
  from scipy.optimize import leastsq
  
  with h5py.File(args.file, 'r') as a:
      tz = a['/observables/tz/value'][:]
      tz_count = a['/observables/tz_count/value'][:]
      vx = a['/observables/vx/value'][:]
      vx_count = a['/observables/vx_count/value'][:]
  
  vx = vx[-1]
  Lz = len(vx)
  z = np.arange(Lz)+0.5
  
  # Fit the parabolic profile
  def fitfunc(p, t):
      return p[0] * t*(Lz-t) / (Lz/2)**2
  
  def errfunc(p, t, y):
      return fitfunc(p, t) - y
  
  p, success = leastsq(errfunc, [0.1,], args=(z, vx))
  assert success in (1, 2, 3, 4)
  
  print("max_speed = ", p[0])
  
  if args.plot:
      import matplotlib.pyplot as plt
      plt.figure()
      plt.plot(z, vx, 'k-', lw=2)
      plt.plot(z, fitfunc(p, z), 'k--')
      plt.xlabel(r'$z$', fontsize=20)
      plt.ylabel(r'$v_x(z)$', fontsize=20)
      plt.title('velocity')
      plt.show()
  
subject: add program fit_poiseuille for flow simulation