Use temperature_t for temperatures in struct stats_t

Use struct temperature_t for temperatures in struct stats_t and
use get_temperature_string() when printing these temperatures for
statistics and HTML export.

Signed-off-by: Stefan Fuchs <sfuchs@gmx.de>

core/units.h

@@ -108,6 +108,11 @@ typedef struct
 	uint32_t mkelvin; // up to 1750 degrees K (temperatures in K are always positive)
 } temperature_t;
 
+typedef struct
+{
+	uint64_t mkelvin; // up to 18446744073 MdegK (temperatures in K are always positive)
+} temperature_sum_t;
+
 typedef struct
 {
 	int mliter;