Fix printing of max threads used by OpenMP (#293)

- Replace omp_get_num_threads() with opm_get_max_threads() to get the
  max number of threads that can OpenMP use

Author: iomaganaris

coreneuron/mpi/nrnmpi.cpp

@@ -125,7 +125,7 @@ void nrnmpi_init(int nrnmpi_under_nrncontrol, int* pargc, char*** pargv) {
     if (nrnmpi_myid == 0) {
 #if defined(_OPENMP)
         printf(" num_mpi=%d\n num_omp_thread=%d\n\n", nrnmpi_numprocs_world,
-               omp_get_num_threads());
+               omp_get_max_threads());
 #else
         printf(" num_mpi=%d\n\n", nrnmpi_numprocs_world);
 #endif