Combine LArHVCondAlg and LArHVScaleCorrAlg into one cond-algo

Incorporate the functionality of LArHVScaleCorrAlg (formerly known as LArHVScaleCorrTool) into LArHVCondAlg. This way we don't need to store the voltages-per-cell in the ConditionsStore. Saves some 50MBytes on q431.

Because of the different code-path, some of the correction factors end up slighlty differently than before (relative diff about 10E-5). No visible difference in the q431 tests.

cc @pavol and @ssnyder with whom I discussed this change before.