Interpolating Basis with ElementComposite fails

[kinnala]

Only the first component is interpolated; remaining components seem to be ignored.

MWE:

In [1]: from skfem import *                                                                      

In [2]: m = MeshTri()                                                                            

In [3]: basis = Basis(m, ElementTriP1() * ElementTriP1())                                        

In [4]: y = basis.zeros()                                                                        

In [5]: basis.interpolate(y)                                                                     
Out[5]: 
(<skfem DiscreteField object>
   Quadrature points per element: 6
   Number of elements: 2
   Order: 0
   Attributes: grad,
 <skfem DiscreteField object>)

Observe how the second component is empty DiscreteField.

[kinnala]

Due to my laziness, this is also being addressed in #865. This seems to be the fix:

@@ -283,12 +283,12 @@ class AbstractBasis:
         if w.shape[0] != self.N:
             raise ValueError("Input array has wrong size.")
 
-        refs = self.basis[0]
+        refs = self.split(w)
         dfs: List[DiscreteField] = []
 
         # loop over solution components
         for c in range(len(refs)):
-            ref = refs[c]
+            ref = refs[c][1].basis[0][0]
             if ref.is_zero():
                 dfs.append(ref)
                 continue
                 ```