Release v2.1.6

[dahlend]

[v2.1.6]

Added

• Added a new numerical integrator based on Picard-Chebyshev integration. This
  integrator has only been added to the rust backend at this point, until more
  testing can be done and it be made available on the frontend.
• Saving SimultaneousStates to parquet files can now optionally include a column
  containing the TDB JD of when the state information was last updated. This allows
  users to selectively update state vectors only when necessary.
• Added multi-core propagation support to rust backend.
• Added kete_stats as a new rust crate, moving some of the fitting and statistics
  tools that have been in kete into their own crate. This is in support for some
  upcoming changes, and is being used as a test case for breaking up kete into smaller
  crates for easier consumption in the rust ecosystem.

Changed

• Rewrite of NEATM and FRM internal models, remove support for saving files related
  to thermal and optical models.
• Throughout the rust code, Time is being enforced as inputs for functions instead
  of accepting f64 in a large number of places.
• Updated SPHEREx SPICE kernel to include orbit through Dec 10, 2025.

Fixed

• Fixed epoch times in PCK Type 2 frames that were not being converted correctly.
• Fixed bug in the non-gravitational model constructor for dust, where the default
  value for converting from diameter to beta values was missing a 1e-3. This only
  impacted when dust models were constructed using the diameter input.