Refactor load interface

# ✨ Feature Request

Parent issue for refactoring the ANTS load interface. Currently this relies on wrapping the iris load functions with a customised load wrapper. This is too generic, and the call signatures are not documented. Ideally we should have more specific loaders, for each file format.

Start by examining the current ANTS load wrapper and the wrapped iris load functions. Identify the supported file formats and their required call signatures, then define the format-specific loaders and document each interface; done means the generic wrapper is replaced by clear, format-specific loading entry points.

Written by the indexing model from the issue text.