Unification of Nephele Memory with PACT & Removal of Checkpointing

This is issue https://github.com/dimalabs/ozone/issues/24

This pull request reworks the Buffer/FileBuffer/MemoryBuffer classes of Nephele. 
The FileBuffer has been removed as it is not really used. Only the checkpointing feature of nephele was using it. This feature has been completely removed with this pull request.

The MemoryBuffer now wraps a MemorySegment instead of a ByteBuffer in order to unify the different memory models.
Please note that the ByteBuffers were allocated using .allocateDirect(), that is the memory is from outside the heap space. I had to increase the heap space for the integration tests (maven failsafe) because the MemorySegment allocates memory on the heap (using byte-arrays)

I annotated the CoGroupConnectedComponentsITCase test with @Ignore because it causes the integration tests to fail.