Convert calibs to "validityUntilSuperseded" mode

Calibration validity ranges is currently determined through a validity period specified when running ingestPfsCalibs.py, and where there are overlaps the midpoint is used. However, we want to use a mode where calibs are valid from the point the data used to construct them was taken until they are superseded. There appears to be a configuration variable for this: validityUntilSuperseded.