Pull inserted axes through TransformCoords (#760)

This patch adds a simplification to the TransformCoords operation that
removes inserted axes from its input coordinates and reapplies them to
the result. Besides improving the efficiency of the transformation
itself, this simplification also exposes insertion structure to any
operation following it such as PolyVal, which can then benefit from a
similar simplification.

nutils/evaluable.py

@@ -3898,6 +3898,10 @@ def _derivative(self, var, seen):
     def _simplified(self):
         if self._target == self._source:
             return self._coords
+        cax = self.ndim - 1
+        coords, where = unalign(self._coords, naxes=cax)
+        if len(where) < cax:
+            return align(TransformCoords(self._target, self._source, self._index, coords), (*where, cax), self.shape)
 
 
 class TransformIndex(Array):