Add aux_factory HybridLogPressureFactory for atmosphere_hybrid_sigma_ln_pressure_coordinate

[k-a-webb]

Description

Following the addition of a new parametric vertical coordinate in the CF conventions (see PR WCRP-CMIP/Essential-Model-Documentation#396, docs https://cfconventions.org/cf-conventions/cf-conventions.html#_atmosphere_hybrid_sigma_log_pressure_coordinate) for atmosphere_hybrid_sigma_ln_pressure_coordinate, a new iris.aux_factory is required to support models with these coordinates.

Please note: This is my first PR to iris, any assistance or helpful feedback is appreciated. I work with the ECCC CCCma on the development of the CanESM models, where CanESM6-0-MR for CMIP7 is one of the datasets that uses atmosphere_hybrid_sigma_log_pressure_coordinate.

The new class, and related modifications, are based on the implementation of the existing HybridPressureFactory.

Checklist

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[k-a-webb]

@JamesAnstey