v2.27.1

Both additions come from @attilayener's derive_statistics work in PR #147: refusing hours without a rate rather than guessing, and showing the old per-month totals next to the new ones. Thank you!

Added

• derive_statistics missing_price: with a price statistic as the factor, missing_price: "refuse" stops the write when any hour in the range has no price, naming how many hours and the first and last. The default, "carry", still takes the last known price for such an hour; the preview's factor now says which was used. Idea and original design by @attilayener (#147).
• derive_statistics month-by-month comparison: when replacing a range of an existing target, the preview's replaces adds by_month: for each month, what the target holds now (before), what it would hold (after), and the difference. Idea and original design by @attilayener (#147).