Fix: implement missing saveScratchPadByIndex/recallScratchPadByIndex

DallasTemperature.h declares saveScratchPadByIndex() and
recallScratchPadByIndex(), and both are documented and used in the
SaveRecallScratchPad example, but their implementations were dropped
from DallasTemperature.cpp during the race-condition refactor merged
in #264. Any sketch calling either method now fails at link time with
"undefined reference to DallasTemperature::saveScratchPadByIndex(...)"
(and the recall counterpart).

Restore the two thin wrapper functions that resolve a device index to
its address via getAddress() and delegate to the existing
saveScratchPad()/recallScratchPad() overloads, matching their
pre-refactor behavior.

Author: 94xhn

DallasTemperature.cpp

@@ -237,6 +237,18 @@ void DallasTemperature::writeScratchPad(const uint8_t* deviceAddress, const uint
     }
 }
 
+bool DallasTemperature::saveScratchPadByIndex(uint8_t deviceIndex) {
+    DeviceAddress deviceAddress;
+    if (!getAddress(deviceAddress, deviceIndex)) return false;
+    return saveScratchPad(deviceAddress);
+}
+
+bool DallasTemperature::recallScratchPadByIndex(uint8_t deviceIndex) {
+    DeviceAddress deviceAddress;
+    if (!getAddress(deviceAddress, deviceIndex)) return false;
+    return recallScratchPad(deviceAddress);
+}
+
 bool DallasTemperature::saveScratchPad(const uint8_t* deviceAddress) {
     if (_wire->reset() == 0) return false;