CohortList: `foreach_cohort()`: Return `cohort_period_acq`

Foreach: Return empty structure if no results collected

Cohort: `foreach_orders` -> `foreach_orders_until`

Cohort.foreach_orders_sincealive() & CustomerTime

Various improvements

CohortList.plot_periods_stackedbar()

OrderSummary & Cohort select since alive

Various Improvements

CohortList & DecileList: List-like access with int and slice

Improve CohortSplitter

Cohort & PeriodData: `to_deciles(splitter)`

DecileList: Custom names & select by position

More consistent Basket/OrderSummary creation

Various smaller improvements

"margin"->"profit", "profit_perc"->"perc_profit_margin"