## 2D Histogram allocation

The functions for allocating memory to a 2D histogram follow the style of `malloc` and `free`. In addition they also perform their own error checking. If there is insufficient memory available to allocate a histogram then the functions call the error handler (with an error number of `GSL_ENOMEM`) in addition to returning a null pointer. Thus if you use the library error handler to abort your program then it isn't necessary to check every 2D histogram `alloc`.

Function: gsl_histogram2d * gsl_histogram2d_alloc (size_t nx, size_t ny)
  This function allocates memory for a two-dimensional histogram with nx bins in the x direction and ny bins in the y direction. The function returns a pointer to a newly created `gsl_histogram2d` struct. If insufficient memory is available a null pointer is returned and the error handler is invoked with an error code of `GSL_ENOMEM`. The bins and ranges must be initialized with one of the functions below before the histogram is ready for use.

Function: int gsl_histogram2d_set_ranges (gsl_histogram2d * h, const double xrange[], size_t xsize, const double yrange[], size_t ysize)
  This function sets the ranges of the existing histogram h using the arrays xrange and yrange of size xsize and ysize respectively. The values of the histogram bins are reset to zero.

Function: int gsl_histogram2d_set_ranges_uniform (gsl_histogram2d * h, double xmin, double xmax, double ymin, double ymax)
  This function sets the ranges of the existing histogram h to cover the ranges xmin to xmax and ymin to ymax uniformly. The values of the histogram bins are reset to zero.

Function: void gsl_histogram2d_free (gsl_histogram2d * h)
  This function frees the 2D histogram h and all of the memory associated with it.