Add new parameters to the detector section related to exposure and live times

New parameters are need for:

preset_time - Desired measuring time
live_time - Time the detector has been actually measuring (elapsed_time - dead_time)
elapsed_time - Time elapsed between start and stop of the measurement
calibration - For MCA detectors, coefficients a, b, c to compute a scale based on channel number as a + b * x + c * x * x