Get rid of MPI compilation warnings

[astrofrog]

These warnings appear during compilation:

Warning: Rank mismatch between actual argument at (1) and actual argument at (2) (scalar and rank-1)
src/mpi/mpi_io.f90:781:19:

  781 |     call mpi_bcast(array, n1*n2, mpi_integer4, rank_main, mpi_comm_world, ierr)
      |                   1
......
 1604 |     call mpi_bcast(n1, 1, mpi_integer4, rank_main, mpi_comm_world, ierr)
      |                   2
Warning: Rank mismatch between actual argument at (1) and actual argument at (2) (scalar and rank-2)
src/mpi/mpi_io.f90:764:19:

  764 |     call mpi_bcast(array, n1, mpi_integer4, rank_main, mpi_comm_world, ierr)
      |                   1
......
 1604 |     call mpi_bcast(n1, 1, mpi_integer4, rank_main, mpi_comm_world, ierr)
      |                   2
Warning: Rank mismatch between actual argument at (1) and actual argument at (2) (scalar and rank-1)
src/mpi/mpi_io.f90:735:19:

We fix this by defining an explicit interface for these subroutines.