[Dune] Assignment operator in DiscreteFunctionDefault

[Adrian_Burri]

Hi,

I stumbled on a problem in the DiscreteFunction implementation and would 
like to suggest a remedy.

DiscreteFunctionDefault has two methods for assignment:
1.) virtual Vector<...>& operator= (const Vector<...>&), inherited from 
Vector<...>
2.) Vector<...>& assign(const Vector<...>&), defined in 
DiscreteFunctionDefault

Both are potentially dangerous, because they allow for mixed type 
assignments (which might be a feature and not a bug here; however, there 
is no means to copy variables defined in subclasses in that case) and 
using a static_cast to DiscreteFunctionDefault in the implementation.

Considering this, I suggest to ban operator= altogether (defining one, 
but protected) on DiscreteFunctionDefault (and on Vector as well) and 
replace assign with a method assignDofs, which does exactly what it has 
done until now: it copies the value of the dofs from one function to 
another. (Note that this operation makes only sense when both 
DiscreteFunction refer to the same DiscreteFunctionSpace. This will be 
checked for in the method.)

This solution has the following benefits
1.) Mixed type assignments are possible where it makes sense, with the 
guarantee that only dofs are copied
2.) Potentially dangerous and unwanted assignments are banned
3.) If someone wants to come up with an assignment operator on a leaf 
class, he can still write one

Please send me comments, suggestions, objections, whatever!

Cheers

Adi