title: zero division error in python uncertainties package
body: <p>Why does the following zero division error occur?</p>
  
  <pre><code>&gt;&gt;&gt; from uncertainties import ufloat
  &gt;&gt;&gt; a = ufloat((0,0))
  &gt;&gt;&gt; x = ufloat((0.3,0.017))
  &gt;&gt;&gt; a**x
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/uncertainties/__init__.py", line 601, in f_with_affine_output
      if arg.derivatives
    File "&lt;string&gt;", line 1, in &lt;lambda&gt;
  ZeroDivisionError: 0.0 cannot be raised to a negative power
  &gt;&gt;&gt; 0.0**x
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/uncertainties/__init__.py", line 601, in f_with_affine_output
      if arg.derivatives
    File "&lt;string&gt;", line 1, in &lt;lambda&gt;
  ValueError: math domain error
  </code></pre>
  
  <p>Shouldn't these both just return <code>0.0</code>?</p>