Add time unit arguments in lead/lag function and arx_args_list() and elsewhere, where necessary

[rachlobay]

From the discussion in PR # 116,

1. Adjust the lead/lag function to accept a time_unit argument (days, months, weeks, years) and perform the operations appropriately.
2. Add an arg to arx_args_list() and elsewhere, where necessary (for ex., consider step_epi_ahead and step_epi_lag), to allow a user to specify the units (possibly different for lags/ahead/train_window).