get_rebalance_dates determines rebalancing dates based on rebalancing
frequency chosen by the user. This is a helper function used by
backtest_allocation and is not intended to be called directly by the user.
get_rebalance_dates(dates, reb_freq, k = 1)A vector of dates.
A vector of dates
Character with rebalancing frequency. Options are
"days", "weeks", "months", "quarters",
and "years"
An integer with number of periods to skip.