title: Use fill_value outside boundaries with scipy.interpolate.griddata and method=nearest
body: <p>Function <a href="https://docs.scipy.org/doc/scipy/reference/generated/scipy.interpolate.griddata.html" rel="nofollow noreferrer"><code>scipy.interpolate.griddata</code></a> allows to specify the keyword <code>fill_value</code>, for which the doc states:</p>
  
  <blockquote>
    <p>Value used to fill in for requested points outside of the convex hull of
    the input points. If not provided, then the default is nan. 
    <strong>This option has no effect for the ‘nearest’ method.</strong></p>
  </blockquote>
  
  <p>However, I need to specify a <code>fill_value</code> to use outside of the boundaries when
  using <code>method='nearest'</code> with 2D data. How can this be achieved?</p>