Fix error in balanceOctree(...)

- include/mpi_tree.txx: In balanceOctree(...), use new_size and new_rank
  after MPI_Comm_split(...)

- include/interac_list.txx: Fix error: more than one instance of
  overloaded function "pow" matches the argument list.

- include/kernel.hpp: include files vector.hpp and matrix.hpp

- include/mortonif.txx: declare max_int as static in contructor, so that
  it is not computed each time.