[PLAT-6858] Added productInterpolator1D and test
[PLAT-6858] Spelling
[PLAT-6860] NaturalCubicSplineInterpolator1D erroneously reports zero gradient at far right of range
[PLAT-6859] DoubleQuadraticInterpolator1D erroneously reports zero gradient at far right of range
[PLAT-6861] Zero clamped left extrapolator
[PLAT-6858] Resolve failing tests, revert linear extrapolator
[PLAT-6858] Rebuild product interpolator
[PLAT-6858] Added unit tests, fixed bugs
[PLAT-6858] Revert changes in polynomial function
[PLAT-6858] Logger added, old classes removed
[PLAT-6858] Revert irrelevant changes in data bundle
[PLAT-6858] added javadoc, extended error tests, deleted unused code
[PLAT-6858] Replaced private methods by InterpolatorTestUtil
Merge pull request #284 from OpenGamma/topic/PLAT-6858

topic/PLAT-6858 Implement ProductPiecewisePolynomialInterpolator and its wrappers.