[Dune] Interface of PersistentContainer

[Christian Engwer]

Dear all,

I wanted to implement the PersistentContainer for YaspGrid and
stumbled upon some questions/issues regarding the interface. 

a) The "official" implementation has a method "size()". What is the
   idea for this method? As it is not possible to access the data in a
   vector like style, I don't understand the purpose. Depending on the
   data storage stragtegy it can be quite expensive to obtain this
   information.
b) what is the desired semantic if I try to access an entry which is
   not stored? E.g. I store the old soluation and access cell-entries
   after a refine, without adjusting the size of the container?

Christian