Provide general filtering infrastructure

# ✨ Feature Request

We should provide a general interface in core ANTS to make it easier for users to develop more filters in contrib.

This may also include a tidy-up of the migrated code, in particular renaming/moving the private `_raymond.py` module in contrib.

## Requirements

This will probably take the form of an abstract base class, following a similar pattern to that used in UG-ANTS.

## Context

This should provide the basis for the existing Raymond filter in contrib, and also allow the development of further filters in contrib.

Start by reading the existing Raymond filter in contrib and the private contrib/_raymond.py module, then compare its structure with the abstract-base-class pattern used in UG-ANTS. Define the general core ANTS filtering interface, tidy the migrated Raymond code if needed, and confirm it supports Raymond and future contrib filters.

Written by the indexing model from the issue text.