krigekit.variogram_accessors

Internal 1-based accessors for the multivariable variogram system.

These wrap the system’s per-pair storage so that the public surface (system.vgm[ivar, jvar]) reads like a symmetric matrix while keeping the mapping mechanics out of VariogramSystem. The accessor never imports the system; the system constructs it and supplies index-validation/growth and storage callbacks.

A pair is materialized once it has a stored object and configured only once that object has at least one component. Internal algorithms must select work by configured_pairs(), never by materialization, so that a harmless read such as system.vgm[1, 2] cannot turn an empty pair into an LMC input.