Replace decimal year conversions with julian year conversions. - #382

This reverts commit c30d2e8.

Actually use pool in ptemcee

periodic_save_freq previously re-derived the entire chain history (including
an unvectorized per-sample prior recomputation) and rewrote the whole HDF5
output file on every periodic save, making cost grow with total run length
instead of just the new chunk. _update_chains_from_sampler now only
processes steps since the last save, and Results.save_results appends to
resizable HDF5 datasets instead of rewriting the file each time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

… saves

Results.add_samples() used np.vstack/np.append, which reallocate and copy
the entire accumulated post/lnlike arrays on every call. This meant that
even after the earlier fix to periodic_save_freq (only recomputing/writing
new chunks), the in-memory accumulation itself was still O(total rows so
far) per periodic save, making the MCMC fit progressively slower as it ran.

add_samples now writes into a capacity-doubling internal buffer, so appends
are amortized O(1) per row instead of O(n) per call.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>