support for CudaUnifiedMemory

This branch originates from a painful attempt to test HYPRE 2.16 GPU support together with @markadams4 and @BarrySmith The HYPRE branch is available, but currently configure options for the repository are disabled.

The good outcome of going through all of the pain of getting HYPRE compiled is that we can now switch at run-time between malloc types

The previous horrible ifdefs in the malloc, realloc and free routines have been removed. This is probably too late to get into the release, up to @BarrySmith to decide.