Refactoring setup for multisend method of spike exchange (#273)

* cleanup of multisend_setup
* use less raw pointers, more vectors
* use scoped variables, range based for loops
* use ofstream than C file I/O

Author: Nicolas Cornu

coreneuron/mpi/mpispike.cpp

@@ -277,7 +277,7 @@ extern void nrnmpi_int_alltoall(int* s, int* r, int n) {
     MPI_Alltoall(s, n, MPI_INT, r, n, MPI_INT, nrnmpi_comm);
 }
 
-extern void nrnmpi_int_alltoallv(int* s, int* scnt, int* sdispl, int* r, int* rcnt, int* rdispl) {
+extern void nrnmpi_int_alltoallv(const int* s, const int* scnt, const int* sdispl, int* r, int* rcnt, int* rdispl) {
     MPI_Alltoallv(s, scnt, sdispl, MPI_INT, r, rcnt, rdispl, MPI_INT, nrnmpi_comm);
 }
 

coreneuron/mpi/nrnmpidec.h

@@ -94,7 +94,7 @@ extern void nrnmpi_int_gatherv(int* s, int scnt, int* r, int* rcnt, int* rdispl,
 extern void nrnmpi_int_allgather(int* s, int* r, int n);
 extern void nrnmpi_int_allgatherv(int* s, int* r, int* n, int* dspl);
 extern void nrnmpi_int_alltoall(int* s, int* r, int n);
-extern void nrnmpi_int_alltoallv(int* s, int* scnt, int* sdispl, int* r, int* rcnt, int* rdispl);
+extern void nrnmpi_int_alltoallv(const int* s, const int* scnt, const int* sdispl, int* r, int* rcnt, int* rdispl);
 extern void nrnmpi_dbl_allgatherv(double* s, double* r, int* n, int* dspl);
 extern void nrnmpi_dbl_alltoallv(double* s,
                                  int* scnt,