title: ValueError: diff requires input that is at least one dimensional
body: <p>I studying how to solve differential equations in Python with <code>odeint</code> and for test, I try to solve the following ODE (the following example came from of <a href="https://apmonitor.com/pdc/index.php/Main/SolveDifferentialEquations" rel="nofollow noreferrer">https://apmonitor.com/pdc/index.php/Main/SolveDifferentialEquations</a>):</p>
  
  <pre><code># first import the necessary libraries
  import numpy as np 
  from scipy.integrate import odeint
  
  # function that returns dy/dt
  def model(y,t):
      k = 0.3
      dydt = -k*y
      return dydt
  
  #Initial condition
  y0 = 5.0
  
  # Time points
  t = np.linspace(0,20)
  
  # Solve ODE
  def y(t):
      return odeint(model,y0,t)
  </code></pre>
  
  <p>So if I plot the results with matplotlib, or more simply, give the command <code>print(y(t))</code> then this work perfectly! But if I try compute the value of the function for a fixed value of time, for instance, <code>t1 = t[2]</code> ( = 0.8163 ) so I get the error</p>
  
  <pre><code>t1 = t[2]
  print(y(t1))
  
  ValueError("diff requires input that is at least one dimensional")
  </code></pre>
  
  <p>why I only can compute the value for <code>y(t)</code> for a interval <code>t = np.linspace(0,20)</code> but not for a number in this interval? There is some manner to fix this? 
  Thank you very much.</p>