CaloCondBlobDat: Add optimized accessor

I was always wondering why the CaloCellTimeCorrTool and the CaloCellEnergyRescaler were relatively slow, eg not faster that the tool actually creating the CaloCellContaiener even though these correction tool are mathematically trivial.

It turns out, that the slowliness comes from the internals of the CaloCondBlob class-hierarchy. This MR introduces a optimized accessor to be used for corrections that do not depend on gain and adc.

This reduces the execution time of CaloCellTimeCorrTool and CaloCellEnergyRescaler from about 10s to about 3s on 1000 events. Keep in mind that these tools are executed for every LAr cell, eg 182468 times per event.