#11172: Decline curve improvements

[magnesj]

Summary

Addresses items 2–6 of #11172.

• Move decline curve math to RigDeclineCurveCalculator, with unit tests
• Add regression of the history section as an alternative to start/end point fit
• History part is a backward extrapolation of the fitted curve
• Time selection by date in addition to sliders; works with multi-selection
• Show equations in plot overlays (configurable corner) and in Show Plot Data export
• Stacked overlay frames in all four plot corners
• New decline curves get distinct colors, solid lines and a legend in project tree order
• New automatic Arps decline curve type that fits the decline parameters from the data, based on the AutoDCA algorithm; includes unit tests

Closes #11172.