utils.py/calibration_and_holdout_data: Add freq_multiplier
utils.py/calibration_and_holdout_data: Add freq_multiplier

Added freq_multiplier to calibration_and_holdout_data.
Also updated 'freq' definition with the correct link for possible values and 'freq_multiplier' definition to make it clearer.

CamDavidsonPilon/lifetimes