old_contents: 
  import numpy as np
  import pyh5md
  import matplotlib.pyplot as plt
  
  f = pyh5md.H5MD_File('particles_3d.h5', 'r')
  com = f.observable('center_of_mass')
  plt.plot(com.time, com.value)
  plt.show()
  
new_contents: 
  import numpy as np
  import pyh5md
  import matplotlib.pyplot as plt
  
  f = pyh5md.H5MD_File('particles_3d.h5', 'r')
  
  rn = f.observable('random_number')
  print rn[()]
  
  com = f.observable('center_of_mass')
  plt.plot(com.time, com.value)
  plt.show()
  
subject: Add opening of scalar observable to example analyze_h5md.