More basic bugs with children and leaves

[Marie Rognes]

Child abuse is getting increasingly common in the Python layer. Here are some more basic examples that fail with trunk.

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from dolfin import *

# Fail 1
mesh = UnitIntervalMesh(2)
V = FunctionSpace(mesh, "CG", 1)
print V.child()

# Fail 2
u0 = Function(V)
print u0.child()

# Non-fail!
print u0.leaf_node()

# Prints None (Weird, expecting <Mesh ... >)
print mesh.child()