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 *Serial DataCollectors* state it as **optional**.

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)