refactoring of part of main code

// Main Code

int main(int argc, char * argv[])

{

        // Main MPI parameters

	int rank;

	int size;

        // Define main filenames

	FILE * pFile;

	FILE * pFile1;

	FILE * pFilereal;

	FILE * pFileimg;

	// Global filename to be composed

	char filename[1000];

	//char datapath[900] = "/m100_scratch/userexternal/cgheller/gridding/old/data/gauss2_t201806301100_SBL180.binMS/";

	//char datapath[900] = "/m100_scratch/userexternal/cgheller/gridding/newgauss2noconj_t201806301100_SBL180.binMS/";

	//char datapath[900] = "/m100_scratch/userexternal/cgheller/gridding/gauss1_t201806301100_SBL180.binMS/";

	//char datapath[900] = "/m100_scratch/userexternal/cgheller/gridding/newgauss4_t201806301100_SBL180.binMS/";

	//char datapath[900] = "/m100_scratch/userexternal/cgheller/gridding/gauss4_t201806301100_SBL180.binMS/";

	//char datapath[900] = "/m100_scratch/userexternal/cgheller/gridding/hba-8hrs_t201806301100_SBH255i-test.binMS/";

	//

	//char datapath[900] = "/m100_scratch/userexternal/cgheller/gridding/hba-8hrs_gauss4new.binMS/";

	//char datapath[900] = "/m100_scratch/userexternal/cgheller/Lofar/Observations/L798046_SB244_uv.uncorr_130B27932t_146MHz.pre-cal.binMS/";

	// MS paths

	char datapath[900];

	char datapath_multi[NFILES][900];

        // Bin MS files

	char ufile[30] = "ucoord.bin";

        char vfile[30] = "vcoord.bin";

        char wfile[30] = "wcoord.bin";

        char visrealfile[30] = "visibilities_real.bin";

        char visimgfile[30] = "visibilities_img.bin";

        char metafile[30] = "meta.txt";

        // Mesh related files 

	char outfile[30] = "grid.txt";

	char outfile1[30] = "coords.txt";

	char outfile2[30] = "grid_real.bin";

	char srank[4];

	char timingfile[30] = "timings.dat";

	// Visibilities related variables

	double * uu;

	double * vv;

	double * ww;

	double wmax,wmax0;

	double resolution;

  // MESH SIZE

        // Mesh related parameters: global size

	int grid_size_x = 2048;

	int grid_size_y = 2048;

	int local_grid_size_x;// = 8;

	int local_grid_size_y;// = 8;

	// Split Mesh size (auto-calculated)

	int local_grid_size_x;

	int local_grid_size_y;

	int xaxis;

	int yaxis;

	// Number of planes in the w direction

	int num_w_planes = 8;

	// DAV: the corresponding KernelLen is calculated within the wstack function. It can be anyway hardcoded for optimization

	// Size of the convoutional kernel support

	int w_support = 7;

	int num_threads;// = 4;

	// Number of OpenMP threads (input parameter)

	int num_threads;

        // Resolution 

	double dx = 1.0/(double)grid_size_x;

	double dw = 1.0/(double)num_w_planes;

	// Half support size

	double w_supporth = (double)((w_support-1)/2)*dx;

	// Internal profiling parameters 

	clock_t start, end, start0, startk, endk;

	double setup_time, process_time, mpi_time, fftw_time, tot_time, kernel_time, reduce_time, compose_time, phase_time;

	double setup_time1, process_time1, mpi_time1, fftw_time1, tot_time1, kernel_time1, reduce_time1, compose_time1, phase_time1;

	double writetime, writetime1;

	struct timespec begin, finish, begin0, begink, finishk;

	double elapsed;

	long nsectors;

  /* GT get nymber of threads exit if not given */

        // Number of sectors in which the mesh is divided along the v direction

	// IF nsectors < 0, nsectors = NUMBER OF MPI RANKS

	long nsectors = NUM_OF_SECTORS;

        /* Get number of threads, exit if not given */

        if (argc == 1) {

          fprintf(stderr, "Usage: %s number_of_OMP_Threads \n", argv[0]);

          exit(1);

        // Set the number of OpenMP threads

        num_threads = atoi(argv[1]);

  if ( num_threads == 0 )

  {

        // Number of threads <= 0 not acceptable

        if (num_threads <= 0) {

           fprintf(stderr, "Wrong parameter: %s\n\n", argv[1]);

           fprintf(stderr, "Usage: %s number_of_OMP_Threads \n", argv[0]);

           exit(1);

        }

	// Internal profiling

	clock_gettime(CLOCK_MONOTONIC, &begin0);

	start0 = clock();

	// Intialize MPI environment

#ifdef USE_MPI

	// Intialize MPI environment

	MPI_Init(&argc,&argv);

	MPI_Comm_rank(MPI_COMM_WORLD, &rank);

	MPI_Comm_size(MPI_COMM_WORLD, &size);

	if(rank == 0)printf("Running with %d MPI tasks\n",size);

  #ifdef USE_FFTW

	// Initialize FFTW environment

	fftw_mpi_init();

  #endif

#else

	// If MPI is not used the two parameters are set for consistency

	rank = 0;

	size = 1;

#endif

	// set the local size of the image

	local_grid_size_x = grid_size_x;

	nsectors = NUM_OF_SECTORS;

	if (nsectors < 0) nsectors = size;

	local_grid_size_y = grid_size_y/nsectors;

	//nsectors = size;