title: Use fixed exponent in scientific notation
body: <p>Consider the following Python snippet:</p>
  <pre><code>for ix in [0.02, 0.2, 2, 20, 200, 2000]:
      iss = str(ix) + &quot;e9&quot;
      isf = float(iss)
      print(iss + &quot;\t=&gt; &quot; + (&quot;%04.03e&quot; % isf) + &quot; (&quot; + str(isf) + &quot;)&quot;)
  </code></pre>
  <p>It generates the following output:</p>
  <pre class="lang-none prettyprint-override"><code>0.02e9  =&gt; 2.000e+07 (20000000.0)
  0.2e9   =&gt; 2.000e+08 (200000000.0)
  2e9     =&gt; 2.000e+09 (2000000000.0)
  20e9    =&gt; 2.000e+10 (20000000000.0)
  200e9   =&gt; 2.000e+11 (2e+11)
  2000e9  =&gt; 2.000e+12 (2e+12)
  </code></pre>
  <p>Is it possible to &quot;go back&quot; somehow? That is:</p>
  <pre class="lang-none prettyprint-override"><code>2.000e+07 =&gt; 0.02e9 
  2.000e+08 =&gt; 0.2e9
  2.000e+09 =&gt; 2e9    
  2.000e+10 =&gt; 20e9   
  2.000e+11 =&gt; 200e9  
  2.000e+12 =&gt; 2000e9
  </code></pre>
  <p>... I'd specify I want the exponent to be <code>e+09</code>; and then whatever number I throw at this hypothetic function, returns the number value in that exponent? Would it be possible to specify zero padding for both the whole and the decimal part in each case? (i.e. <code>000.0200e9</code> and <code>020.0000e9</code>)?</p>