Define and correctly spell SabrParametricVolatility calibration-grid axis accessors

[mattmenefee]

Summary

SabrParametricVolatility declares two accessors for its calibration-grid axes in qle/termstructures/sabrparametricvolatility.hpp:

// the calculated grid of option expiries and the underlying lenghts
const std::vector<Real>& timeToEpiries() const;
const std::vector<Real>& underlyingLenghts() const;

Neither is defined in sabrparametricvolatility.cpp or anywhere else in the tree, so calling either one fails at link time with an undefined symbol. This PR:

• Defines both accessors to return the existing timeToExpiries_ / underlyingLengths_ members.
• Corrects the misspelled names — timeToEpiries → timeToExpiries, underlyingLenghts → underlyingLengths — along with the lenghts typos in the two adjacent comments.

Why this matters

The class publicly exposes its calibration results as matrices — alpha(), beta(), nu(), rho(), lognormalShift(), numberOfCalibrationAttempts(), calibrationError(), isInterpolated() — laid out as rows = underlying lengths, cols = option expiries. Without the axis vectors, a caller cannot map a matrix cell back to the (timeToExpiry, underlyingLength) node it was calibrated at, which makes the matrices unusable for per-pillar calibration diagnostics or reporting.

On the rename

Renaming a public accessor would normally be a source-breaking change. It is not one here: since neither function was ever defined, no code could have linked against the old spellings. Correcting them now is free; after the definitions ship it stops being free.

Happy to drop the rename and keep the original spellings if you would rather preserve the declared names verbatim — the definitions are the substantive part of the change.

Test plan

• QuantExt compiles and links cleanly with the change
• Both symbols are present in the resulting libQuantExt binary
• Repo-wide grep confirms no other references to the old spellings existed