title: type numpy.ndarray doesn't define __round__ method
body: <p>this is the code:</p>
  
  <pre><code>import numpy as np
  def f_func(state,time,cd,mass,rho,A):
      """Calculate the differential of state vector as a function of time
  
          Args:
          state (list): the state vector at time t
          time (float): the time t
          cd (float): the dimensionless drag coefficient
          mass (float): mass of the object in kg
          rho (float): density of air (kg/m3)
          A (float): cross-sectional area of object (kg)
  
          Returns:
          (list): the differential of the state vector at time t
      """
      # defensive program - check shape of state vector
      assert len(state)==2, "Expected length 2 state vector"
      vy,y = state
      # YOUR CODE HERE
  
      X = np.array([[vy],[y]])
  
      # we know d**2 y / d t**2 = a = -g + 1/(2mass)*(cd*rho*A*vy**2)
      d2ydt2 = -g + (1/(2*mass))*(cd*rho*A*vy**2)
      a = d2ydt2
      # WE KNOW d y / d t = vy
      dXdt = np.array([[-g + (1/(2*mass))*(cd*rho*A*(vy)**2)],[vy]])
  
      return dXdt
  </code></pre>
  
  <p>Checked against the following:</p>
  
  <pre><code> from nose.tools import assert_equal, assert_almost_equal
   a,vy = f_func([0.,78.],0.0,0.5,1,1.2,1)
   assert_almost_equal(a, -9.8)
   assert_almost_equal(vy, 0.0)
   a,vy = f_func([-2.,78.],0.0,0.5,1,1.2,1)
   assert_almost_equal(a,-8.6)
   assert_almost_equal(vy,-2) 
   '''
  </code></pre>
  
  <p>The error message, which I don't understand:</p>
  
  <pre><code>type numpy.ndarray doesn't define __round__ method (error from line 6)
  </code></pre>