Interpolator clean up

Created by: pmpowers-usgs

• Addt'l documentation
• Builder.decreasingX is not clear; we're really talking about y-values monotonically increasing or decreasing: "Because we assume (enforce?) x-values are increasing and non-repeating, deriving y-from-x is straightforward. However, when deriving x-from-y the interpolator searches ascending though any supplied x-y data and needs to know if y-values increase or decrease after the fist value."
• Consider public validating methods and private non-validating; may not be possible to access in package without explicit 'nonValidating' method signatures.
• Consider extrapolation flag (only permitted for y-from-x)
• Consider returning NaN instead of 0 for out of range y-from-x targets; if x-values are in log space, then the interpolator is actually returning a value of 1 (this is currently producing misleading desegregation results for out of range return periods)
• Write tests