stats: Add a way to specify if the data set is a population or a sample

This changes how we compute the variance. We want an unbiased variance
when reasoning about a sample.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>

lib/tests/igt_stats.c

@@ -89,6 +89,7 @@ static void test_std_deviation(void)
 	double mean, variance, std_deviation;
 
 	igt_stats_init(&stats, 8);
+	igt_stats_set_population(&stats, true);
 
 	igt_stats_push(&stats, 2);
 	igt_stats_push(&stats, 4);