Add smileSection() to BlackVolTermStructure

[quantales]

This PR proposes adding smileSection(Date/Time) to BlackVolTermStructure, making EQ/FX vol surfaces in line with the IR ones (OptionletVolatilityStructure, SwaptionVolatilityStructure).

• Public non-virtual smileSection()
• Protected virtual smileSectionImpl(Time) provides a default adapter that wraps blackVol() into a SmileSection
• All existing derived classes would get a working smileSection() with no code changes

Unlike the IR vol structures where smileSectionImpl is pure virtual, the default adapter here means vol-native classes (BlackConstantVol, HestonBlackVolSurface, etc.) would work out of the box. Smile-native classes can override for efficiency: PiecewiseBlackVarianceSurface does so in this PR, returning its stored SmileSection directly at stored tenors. BlackVolatilitySurfaceDelta is another candidate: its existing blackVolSmile() methods are functionally equivalent to smileSection() and could be refactored to override smileSectionImpl() in a follow-up.

Motivation: From the discussions in #2443, #2478, and #2368, and increasing use of SmileSection for EQ/FX vol surfaces (PiecewiseBlackVarianceSurface and BlackVolatilitySurfaceDelta), the base class not returning them felt like a gap (there was no way to get a SmileSection from a PiecewiseBlackVarianceSurface and users needing one from any other BlackVolTermStructure would have to write their own adapter each time). Happy to adjust and would love to hear your thoughts on this.

Tests: default adapter (BlackConstantVol), override at stored tenor with pointer equality (PiecewiseBlackVarianceSurface), adapter fallback between tenors. Full suite passes.

[coveralls]

coverage: 74.351% (+0.002%) from 74.349%
when pulling 5886cf3 on quantales:feature/blackvol-smilesection
into 1c3d908 on lballabio:master.