Units fallback: replace crashing data.<attr> fallback with explicit values or CellMeta

[jepegit]

Deferred from #66 (code review 2026-07, item D4/A2).

units.get_converter_to_specific reads data.raw_units, data.mass, data.active_electrode_area, data.volume; nominal_capacity_as_absolute reads data.nom_cap_specifics, data.nom_cap. The core Data class has none of these attributes, so the fallback path (taken whenever a caller omits specific_converters in add_scaled_summary_columns) raises AttributeError for standalone cellpy-core users.

Make these functions take explicit values or a metadata.CellMeta (which already has mass, active_electrode_area, nom_cap, nom_cap_specifics) — a natural fit with the metadata-boundary decision. Also remove the pointless try/except Exception: raise block in nominal_capacity_as_absolute.

See .issueflows/04-designs-and-guides/code-review-2026-07.md (A2, D4).