fix fluid system unit test

this was due to a missing initialization of the MPI. Because
MPI_Finalize should probably called even if the program is aborted due
to an exception, this is done using a helper class. (which does the
same job as Dune::MPIHelper, but is much simpler.)
Extracts the full compiler path and feeds it to cmake.

Previously, specifying the compiler name with a variable
to configure ("configure CC=gcc") lead to CMake complaining
that <builddir>/$CC was not a valid path. This patch fixes
this by extracting the full path with "which <compiler>".
Should fix issue #355.
substituted which with command -v.
Change which test for cmake28 to use command -v.
Test for executable and preserve erroneous path for messages

If an alternate compiler is specified, then check if this is actually
executable before assigning it so that we don't end up with specifying
an empty name to CMake. If the path does not exist, it will be preserved
so that CMake will complain with an easily identifiable error message.
the big rename

make all file names confirm to the OPM conventions. (hopefully)
remove some lonely usages of Dune::NotImplemented
use the spline class correctly

(i.e., remove a template argument. This only fell through the cracks
because Somerton.hpp is not yet used by a unit test...)