Remove chrono and use hifitime & jiff

[gsleap]

Breaking

• convert_gpstime_to_unixtime and convert_unixtime_to_gpstime (in misc, and re-exported at the crate root) no longer take mwa_start_gpstime_ms/mwa_start_unixtime_ms reference parameters - they are now convert_gpstime_to_unixtime(gpstime_ms: u64) and convert_unixtime_to_gpstime(unixtime_ms: u64). Internally they now convert via hifitime's leap-second table instead of a flat per-observation offset, which also removes the previous assumption that an observation could never span a leap second boundary.
• chrono has been removed entirely. MetafitsContext::sched_start_utc/sched_end_utc and VoltageBeam::modtime are now jiff::Timestamp instead of chrono::DateTime<FixedOffset>. jiff::Timestamp.
  • MetafitsContext's Display output now prints Scheduled start (utc) and Scheduled end (utc) using jiff::Timestamp's own Display, which is genuine RFC3339 (e.g. 2014-12-01T21:08:16Z) - unlike chrono's default Display format (2014-12-01 21:08:16 +00:00, space-separated and not itself RFC3339-compliant). This is a deliberate move to a standards-compliant format.
  • Python: these fields continue to surface as native datetime.datetime objects with no change to the Python-visible type or semantics. jiff::Timestamp already has working IntoPyObject/FromPyObject via pyo3's own jiff-02 feature (the same mechanism chrono previously used).
  • FFI/C: no change - these fields were already exposed as time_t, computed the same way (seconds since the UNIX epoch, now via Timestamp::as_second() instead of chrono's .timestamp()).

Changed

• Added hifitime as a dependency to back the above leap-second-aware conversions.
• Added jiff as a dependency so the field types on MetafitsContext/VoltageBeam are consistent regardless of which features are enabled to back the chrono removal above. hifitime and jiff serve different purposes in mwalib: hifitime for leap-second/GPS-time-scale-aware arithmetic, jiff for the plain UTC timestamp fields that need to interoperate cleanly with Python's datetime.datetime.