title: accessing value of a random variable in PyMC3
body: <p>In PyMC2, there are methods random() and value() to generate a random value, and get the current value of random variables. Is there any way to do the same in PyMC3?</p>
  
  <pre><code>p = pm.Dirichlet('p', theta=np.array([1., 1., 1.]))
  p.random()
  p.value
  </code></pre>