Tomographic SACC writers: one tracer and n(z) per bin #376

D's SACC writers (cosmo_val/sacc_writers.py) are single-bin by construction: BIN = (0, 0), and core.sacc_nz returns {0: n(z)}. The tomography branch measures every statistic per bin pair. So once the branches are merged (#374), a tomographic run has nowhere to write its data vector.

Desired state: one SACC file per catalogue version carrying every tomographic bin:

• one tracer per bin, with that bin's n(z) taken from the multi-column redshift file;
• data points per bin pair for ξ±, pseudo-Cℓ (with bandpower windows), COSEBIs, pure-E/B and ρ/τ;
• covariance blocks in matching order.

The non-tomographic ("all","all") case reproduces today's single-bin files exactly.

This should also absorb the 2PCF→SACC path in #306, extending xi_to_sacc rather than keeping a second writer.

Starts after #374 merges.

— Claude on behalf of Cail