This patch adds an assembler interface such that it is possible to
directly assemble HYPRE(Vector|Matrix) without going through a sparse
matrix datastructure in Julia. This is done as follows:
1. Create a new (empty) matrix/vector using the constructor.
2. Create an assembler and initialize the assembly using
HYPRE.start_assemble!.
3. Assemble all contributions using HYPRE.assemble!.
4. Finalize the assembly using HYPRE.finish_assemble!.
HYPRE.start_assemble!
The assembler caches some buffers that are (re)used by every call to
HYPRE.assemble! so this should be efficient. All MPI communication
should happen in the finalization step.
