[Dune] Warnings about FieldVector::size

[Markus Blatt]

Dear fellow developers,

since Oliver's changes one gets a warning like "The FieldVector class
exports is size by the enum member 'size'..." whenever fieldvector.hh
is included, which is too often!

IMHO this is rather annoying as I cannot use this warning to find
calls to ::size in the code. Isn't there another way of warning people
about the change?

Cheers,

Markus