Function::operator= does not handle the case where "this" is a subfunction.

[Ennio Barbaro]

Consider the code:

Function F,U; //Assume F and U are initialized properly

F[0] = U;

The result of this function is:

F[0]._vector != F._vector.

F[0]._function_space is potentially not a subspace of F._function_space.

No error of any kind are prompted.
operator=(const Function & v) should behave as:

1) Make the assignment illegal if "this" is a subfunction and this->_function_space != v._function_space
2) Overwrite only the relevant entries of this->_vector

At the moment is quite counter-intuitive and also very hard to debug, since if you plot F[0] you obtain the function you just assigned, while F is something completely unrelated.

[Johannes Ring (johannr)]

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