Adding "calibrateCell" function to ICalibrateCaloHitsTool

[SSinngh99]

The CalibrateInLayersTool has a private function called "calibrateCell" that performs a calibration for a single cell. On the k4RecCalorimeter side of things, it would be good to move it from a private member to public. However, this needs a concurrent change to the ICalibrateCaloHitsTool.h, hence the feature request.

[tmadlener]

I don't think anything speaks against moving this to a public member function here. Would you like to open a corresponding PR? That could then also be used for some further discussion if necessary.