anchor.diff:  Get the total number of observations so far made.
   
   If the distribution is `NULL`, then return `0`.
  +
  +*Examples:*
  +[source,c]
  +----
  +inform_dist *dist = NULL;
  +assert(inform_dist_counts(dist) == 0);
  +----
  +[source,c]
  +----
  +inform_dist *dist = inform_dist_uniform(5);
  +assert(dist);
  +assert(inform_dist_counts(dist) == 5);
  +inform_dist_set(dist,2) = 4;
  +inform_dist_set(dist,3) = 3;
  +assert(inform_dist_counts(dist) == 10);
  +inform_dist_free(dist);
  +----
   ****
   
   ****
  
positive.code: implementations of information-theoretic measures possible. Most of the measures implemented
  in *Inform* use this structure at their core.
  
  ****
  [[inform_dist]]
  [source,c]
  
negative.code: In order to safely extract probabilities, the distribution must be non-`NULL`, the size of
  the support must be non-zero and the number of observations must be non-zero. In any other
  case, the distribution is invalid.
  
  *Examples:*
  [source,c]
  ----
  inform_dist *dist = NULL;
  assert(!inform_dist_is_valid(dist));
  ----
  [source,c]
  ----
  inform_dist *dist = inform_dist_alloc(3);
  assert(dist);
  assert(!inform_dist_is_valid(dist));
  inform_dist_free(dist);
  ----
  [source,c]
  ----
  inform_dist *dist = inform_dist_uniform(5);
  assert(dist);
  assert(inform_dist_is_valid(dist));
  inform_dist_free(dist);
  ----
  ****
  
  ****
  