Added API function to retrieve COUNT values at runtime.

The function mcell_get_counter_value retrieves the instantaneous
count value of any COUNT statement defined in the mdl. This
function is bolted on to our current COUNT infrastructure and
retrieves count data via the string name of the output file defined
in the mdl (via a hash). This is perhaps a bit clunky but works for
the time being.

The function can be called anytime during the main iteration
loop to get at the instantaneous counts. A typical invocation
may be

enum count_type_t data_type;
double raw_count = 0;
if (mcell_get_counter_value(world, "<some output file name>", <column>,
  &raw_count, &data_type))
{
  mcell_log("failed to retrieve counter value");
  return 1;
}

if (data_type != COUNT_INT)
{
  mcell_log("Incorrect datatype retrieved - expected int.");
  return 1;
}

int count = (int)raw_count;
if (count == 100) {
  mcell_log("The count of XXX is now 100");
  .... do something .....
}