swev-id: astropy__astropy-14369 — CDS units: correct chained division associativity; fix ascii.cds MRT unit parsing (fixes #73)

[rowan-stein]

This PR fixes incorrect parsing of CDS unit strings with chained division, which affected MRT files read via ascii.cds.

Summary

• Make chained division left-associative in CDS grammar so that A/B/C/D parses as A * B^-1 * C^-1 * D^-1 (consistent with Generic and OGIP formats).
• Continue to support leading inverse (e.g., /pixel/s → (pixel*s)^-1).
• Keep factor/exponent semantics unchanged (10+3J, kpc2).

Tests

• Units (CDS):
  • 10+3J/m/s/kpc2 → 1000 J / (kpc2 m s)
  • 10-7J/s/kpc2 → 1e-07 J / (kpc2 s)
• ascii.cds reader:
  • MRT-style header using the above units; resulting table columns have the correct units.

Implementation notes

• Updated astropy/units/format/cds.py grammar; regenerated PLY tables (cds_lextab.py, cds_parsetab.py).
• Added minimal alias mappings for CDS names (pix/pixel, vox/voxel, photon) to ensure spelled-out forms parse when used with leading division.
• No lexer changes required.
• Parentheses continue to override associativity; existing factor/exponent rules unchanged.

Fixes #73.

Reference: Task swev-id: astropy__astropy-14369.

[noa-lucent]

Thanks for regenerating the CDS parser assets. I verified the grammar changes, the regenerated parsetab matches those productions, the lexer table still exposes the punctuation tokens, and the new CDS/MRT tests cover the left-associative and leading-slash scenarios. The alias additions are minimal and targeted. Approved.