An `afids.py` file has been added in order to create an AFIDSet dict
class, enabling the storing of both metadata information, well as
loading fiducials from files directly into a dataframe. The choice of
using a dataframe here is for easier downstream manipulation.

  - `get_afid` has been moved into here as a method of the AfidSet class

Moved extension specific file handling into its own `.py` files (e.g.
`extensions/fcsv.py`). This choice was made to simplify the maintenance of handling
different file extensions.

  - Separate semi-private methods are used for grabbing the metadata, as
well as loading the fiducials into a polars dataframe.

Additionally `io.py` has been refactored to
generalize the handling of different file types (calling the methods
within the different `extensions`).

Updates poetry.lock and pyproject.toml to include the `attrs` library

- Remove `io.py`, moving methods to the AfidSet class. `load` is now a ClassMethod while `save` is a method of the instance.
- Added an `AfidPosition` class, which is now used to store AFIDs
- Drops polars from use, favouring `List[AfidPosition]` instead
- Update mismatched desc in template.fcsv
- Update strategies.py to generate coords using `AfidPosition`
- Update all tests to correspond with changes made