Feature/12579 squash

Fix ZEN-12579, and part of ZEN-12992
    
Key changes:
    - Add linear interpolation for missing internal points in a series (ZEN-12579)
    - Remove internal timestamp representation that differed from Unix format
    - Return NaN for missing data points (ZEN-12992)
    - Make interpolation optional, and per-series. (ZEN-12992)
    - Code cleanup: fix some issues reported by findbugs and intellij analysis.
    - Remove obsolete unit tests, create new ones for new code and improved coverage
    - Update README to reflect interface changes