[dune-fem] LocalFunctionAdapter

[Agnese, Marco]

Hi Martin,
it worked. Thank you very much for the explanation. 
At the end I realized that I would have to cast x into a DomainType but I wasn't aware of the function coordinate(...)!

I have improved the documentation of the LocalFunctionAdapter with what we have said on the mailinglist to avoid this confusion in the future.

Maybe I will add also (when is finished) my LocalAnalyticalFunction so user can use it to transform a C++ function into a GridFunction in a very simple way. 
Do you think would be useful? 

Cheers,
Marco.