Preprocessing partial fit

The online form of the data steps

A Pipeline is a host of the online step (ADR 0142), and the data steps before its row owner take the step in the form their class allows. A row-local step folds a block of observations and emits the block its transform gives it, through PortfolioOptimisers.partial_fit_transform: PricesToReturns keeps the last price row it saw, PriceGapFill with a CarriedPrice keeps the carried prices, and MissingDataFilter keeps its missing counts. Each reads its fitted Result out through fit_preprocessing with no data. A window-valued configuration answers PortfolioOptimisers.supports_partial_fit false, and the Pipeline refuses it at warm-up by name.