Add Read Cache and Stats

- baro.cache_ttl_ms avoids multiple reads
- by default, cache is off (cache_ttl_ms=0)
- stats are updated (similar to how prometheus-sensors does this)

src/mgos_barometer_mpl115.c

@@ -12,6 +12,7 @@ bool mgos_barometer_mpl115_create(struct mgos_barometer *dev) {
   mpl115_data=calloc(1, sizeof(struct mgos_barometer_mpl115_data));
   if (!mpl115_data) return false;
 
+  // TODO(pim): fix this -- math is wrong.
   mpl115_data->a0=(float)(((uint16_t) data[0] << 8) | data[1]) / 8.0;
   mpl115_data->b1=(float)(((uint16_t) data[2] << 8) | data[3]) / 8192.0;
   mpl115_data->b2=(float)(((uint16_t) data[4] << 8) | data[5]) / 16384.0;