[Dune] HAVE_MPI and it's usage

[Christian Engwer]

Hi,

on friday I commited a change in the semantic of HAVE_MPI:

HAVE_MPI is true if configure found an instance of MPI and if the user
uses MPI (this means he passed the MPI_CPPFLAGS to the compiler).

For the application writer this means, all code that depends on MPI
should be enclosed in an

#if HAVE_MPI
...code...
#endif

statement.

This allows every user to switch between parallel and sequentiell mode
just by changing the compiler flags.

CU Christian