[Dune] nullptr

[Christian Engwer]

Dear all,

we recently experienced problems with implicit casts from 0 to
pointer when using the latest g++ version. Oliver pointed to a
proposal that 0 should not be castable to a pointer. In the same
proposal nullptr was suggested, in order to destinguish between 0 and
a null pointer.

The propsal suggests to add a new keyword, but also provides an
implementation that could be used and would show the complete behavior
of the new keyword. This could be used for a transition.

I wrote an m4 test for nullptr, implemented the proposed class and
added tests. I could commit these changes sothat everybody could have
a look at them and test them.

Comments?

Christian