Helper function: TimeSeries.moving() with specific rebalancing period

[almostintuitive]

Based on https://julialang.zulipchat.com/#narrow/stream/282925-backtesting/topic/Vectorized.20framework/near/232046548

[almostintuitive]

So, instead of using simply TimeSeries.moving(), we'd need a similar function that is only called at specific time periods, like every N days.