Rolling returns, category rank and direct-vs-regular cost

[iamsaquib8]

What

Roadmap item "Mutual fund depth". mf shows NAVs, SIPs and expense ratios, but not the numbers people compare funds on.

Design

• Rolling 1/3/5-year returns from AMFI NAV history: median, min, max and the share of periods that beat the category or a benchmark index fund.
• Category rank by rolling return and by expense ratio, within AMFI's category.
• Direct vs regular: the cost of the regular plan's extra TER over 10 and 20 years for a given SIP (--sip 10000).
• stockwise mf compare <fund> <fund> and --json.

Done when

• Rolling returns match a hand calculation on a fixture NAV series.
• Direct-vs-regular cost is shown for any fund with both plans.

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Moved from issue #250 to keep the issue tracker for scheduled work. Upvote or comment if you would use this; ideas with demand go back on the roadmap.