Assign operator should not return reference to python

[Martin Sandve Alnæs]

a.operator=(b) for functions is mapped to python a.assign(b), and this returns a reference (to a/this). This reference is not properly wrapped in python, but I think it should rather be ignored than returned anyway.

from dolfin import *
c = Constant(0)
d = c.assign(Constant(1))

print d.__class__.mro()
"""
[<class 'dolfin.cpp.Constant'>,
 <class 'dolfin.cpp.Expression'>,
 <class 'dolfin.cpp.GenericFunction'>,
 <class 'ufc.ufc.function'>,
 <class 'dolfin.cpp.Variable'>,
 <type 'object'>]
"""

print c.__class__.mro()
"""
[<class 'dolfin.functions.constant.Constant'>,
 <class 'ufl.coefficient.Coefficient'>,
 <class 'ufl.terminal.FormArgument'>,
 <class 'ufl.terminal.Terminal'>,
 <class 'ufl.expr.Expr'>,
 <class 'ufl.common.Counted'>,
 <class 'dolfin.cpp.Constant'>,
 <class 'dolfin.cpp.Expression'>,
 <class 'dolfin.cpp.GenericFunction'>,
 <class 'ufc.ufc.function'>,
 <class 'dolfin.cpp.Variable'>,
 <type 'object'>]
"""