Add functions for correcting non-linear PMT signal

[153957]

Add PMT saturation (also affected by ADC saturation) curve to signal_calibration module.
Allows for the correction of high signal values to the correct (higher) value.
Rename current calibration module to timing_calibration, since it only deals with timing issues.

Todo:

• Provide good starting values for the parameters
• Add tests for the new functions
• Add function to fit data? (actually only good for fitting data with known inputs, i.e. KASCADE and LED tests)
• Use this in reconstructions/processing (Correct particle count for PMT linearity #55)
• Use this in simulations (Add PMT non linearity curve to simulation #97)
• Determine PMT type per detector (over time):
  non-linear or new linear (still affected by ADC saturation)