Fix: implement missing saveScratchPadByIndex/recallScratchPadByIndex

[94xhn]

Problem

DallasTemperature.h declares saveScratchPadByIndex(uint8_t) and recallScratchPadByIndex(uint8_t), and both are referenced in the SaveRecallScratchPad example, but their implementations are missing from DallasTemperature.cpp. They appear to have been dropped during the race-condition refactor merged in #264 (the header declarations, keywords.txt entries, and example comments survived the refactor; the two .cpp bodies did not).

Any sketch that calls either method now compiles fine but fails at link time:

undefined reference to `DallasTemperature::saveScratchPadByIndex(unsigned char)'
undefined reference to `DallasTemperature::recallScratchPadByIndex(unsigned char)'

Fix

Restore the two thin wrapper functions (same logic as before the refactor): resolve the device index to its address via getAddress(), then delegate to the existing saveScratchPad() / recallScratchPad() address-based overloads.

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);
}

The change is purely additive — no other lines in the file are touched, and no header changes are needed since the declarations are already correct.

Testing

Built a small host-side harness (stub Arduino.h/OneWire.h + a DallasTemperature instance with a mocked single-device bus) and confirmed:

• Current master (unmodified): compiles, fails to link with the undefined-reference errors above.
• With this patch: compiles, links, and runs; saveScratchPadByIndex(0) / recallScratchPadByIndex(0) both return true for a valid index and false for an out-of-range index (matching getAddress()'s failure propagation), with no change to any other existing behavior (getResolution(), isParasitePowerMode(), etc. all unaffected).