'Performs the interpolation at points belonging to the triangulation
(inside an unmasked triangles).
Parameters
return_index : string key from {\'z\', \'dzdx\', \'dzdy\'}
Identifies the requested values (z or its derivatives)
tri_index : 1d integer array
Valid triangle index (-1 prohibited)
x, y : 1d arrays, same shape as `tri_index`
Valid locations where interpolation is requested.
Returns
ret : 1-d array
Returned array of the same size as *tri_index*'

def _interpolate_single_key(self, return_key, tri_index, x, y):

    raise NotImplementedError((u'TriInterpolator    subclasses' + u'should    implement    _interpolate_single_key!'))
