[dune-pdelab] ISTLBlockVectorContainer::size() method

[Oleh Krehel]

Dear PDELab developers,

It appears that ISTLBlockVectorContainer has no size method,
although it wraps around BlockVector, which does.
Instead it wraps BlockVector's N() method, which does exactly the same
thing(currently) as size().
Please add the size() method to ISTLBlockVectorContainer,
and possibly deprecate N() method in BlockVector, unless it serves some purpose.

best regards,
Oleh Krehel