Fix ZEN-12579, and part of ZEN-12992

Key changes:
- Add linear interpolation for missing internal points in a series
- Remove internal timestamp representation that differed from Unix format
- Return NaN for missing data points
- Make interpolation optional, and per-series.
- 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