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

From the discussion in [PR # 116](https://github.com/cmu-delphi/epipredict/pull/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).