Add Downloader and CMORiser for IAPv4.2 ocean temperature and ocean heat content gridded dataset

[rbeucher]

To enhance the evaluation of ocean metrics, we at ACCESS-NRI are working on integrating key observational and reanalysis datasets. As part of the ongoing evaluation of ACCESS-OM3, we have identified several relevant observational datasets.

I plan to submit a PR to introduce a downloader and CMORiser for the IAP ocean temperature and ocean heat content gridded dataset. This dataset offers global ocean temperature data at 1°×1° horizontal resolution, spanning 0–6000m depth (119 levels) with monthly records from 1940 to the present. It includes advanced bias corrections, quality control, and mapping improvements (Cheng et al. 2024), making it highly suitable for studies of climate variability and change across a range of spatial and temporal scales.