The classes FMpiBufferWriter and FBufferWriter implemented the same
functionality, FMpiBufferReader and FBufferReader did too. This replaces
the FBuffer implementations with the FMpiBuffer ones.

Steps taken:
  - remove FBuffer* implementation
  - rename FMpiBuffer* to FBuffer*
  - clean up code and documentation
  - fix missing includes

A few unused variables warnings are fixed in FAbstractBuffer.hpp.

There was a discrepancy in the getSize method of F(Mpi)BufferReader. The
behaviour of FMpiBufferReader is kept and the unit test is modified.