ntpstats 3.6.0

The 2012 thesis, finished (#40). No breaking changes.

Added

• Clock disciplines in the bench (ntpstats.disciplines), steered in closed loop like the PTP
  servos: ntpd (ntpd 4.2.8's clock filter, state machine and hybrid PLL/FLL, from
  ntp_loopfilter.c), ntpd-rfc (the same with the RFC 5905 appendix's constants), lockclock
  (J. Levine's NIST frequency-lock loop, J. Res. NIST 2020) and levine-kalman (with Levine's scalar
  Kalman time estimate, PTTI 2011). The appendix's PLL gain of 65536 gives a phase time constant of
  about 48 days at poll 6; implementations follow ntpd's 16 (Trace replay & PTP).
• ntpstats poll and poll_advice(): the predicted error just before the next poll for each
  candidate interval (the log decimated to it, then the holdover model), and the longest interval
  that meets --target (exit code 3 if none) (Metrology).
• research/thesis-2012: the 2012 thesis assessed and its experiments re-run, now with the ntpd
  model's step and ramp responses and the poll-interval advice for the 2012 clock.

Stable API changes since 3.5.0

• Added: ntpd_discipline, lockclock, poll_advice, PollAdvice. Nothing removed or changed.