Build Matrix for Serial and Parallel Version

The idea of this pull-request is to de-clutter the dependencies
- Serial HDF5 <-> Parallel HDF5
- MPI <-> serial program

In principal, it is possible to compile and test libSplash **without MPI**. The *Parallel DataCollectors* **require** MPI and *the serial version* states it as **optional** (for tools and tests *only*!)

I extended the `example/` folder to compile with and without MPI.

Unfortunately, the `tests/` do not support compiling without MPI yet. That's a bigger change to be implemented in the near future which would allow:
- compiling and testing libSplash *without* MPI at all

Current state:
- compiling serial version without MPI (ok)
- testing serial version without MPI ~~(no)~~ (ok)

P.S.: I am sorry for the white-space mess I caused. Add `?w=1` to the URL during the [diff-view](https://github.com/ComputationalRadiationPhysics/libSplash/pull/12/files?w=1) to ignore them.
P.S.: One other thing I noticed. One can *not trust* on the return value of mpirun. Sometimes, e.g. during MPI_Init() errors, it does write to stdout, but it does not return a non-zero exit code.