Per-projection geometry cone3D_Flex - #2039

Changes

This is adding per-projection geometry.

Building on @effepivi's work in #1850

There are many CIL features that aren't implemented for this geometry in this PR, but will be in the future. The main change is a new geometry typle Cone3D_Flex with dimension labels 'channel' 'projection' 'vertical' 'horizontal'. This PR adds the CIL API for Cone3D_Flex and enables the astra projectors and FDK to run on this geometry type.

Tigre support will be added as a followup PR.

The main changes are:

Wrappers/Python/cil/framework/acquisition_geometry.py

1. Adds a cone3D_Flex acquisition data type
2. Adds factory create_Cone3D_Flex method to create datatype
3. Updates some behaviour of acquisition geometry members/methods to account for it
4. Updates to docstrings of each AcquisitionGeometry factory for consistency.
5. Updates get_slice for either set of dimension labels

Wrappers/Python/cil/framework/labels.py

1. Adds cone_flex to the AcquisitionType labels
2. Adds projection as the dimension label for ConeFlex in place of angle

Wrappers/Python/cil/plugins/astra/utilities/convert_geometry_to_astra_vec_3D.py

1. Updates to set up astra geometry from cone flex

Wrappers/Python/cil/io

1. NexusDataWriter NotImplemented

Wrappers/Python/cil/utilities/display.py

1. Added new display method for cone flex data show_system_positions
2. Show2D can be used when slicing on projections

Wrappers/Python/cil/processors

1. Slicer allows slicing in projection direction only
2. Binner/Padder raise NotImplemented
3. CofR corrector raises error, not supported as no rotation axis defined.
4. FluxNormaliser NotImplemented
5. Paganin NotImplemented due to geometry.magnification

Testing you performed

Please add any demo scripts to https://github.com/TomographicImaging/CIL-Demos/tree/main/misc

Notebook here demonstrates use:
https://github.com/TomographicImaging/CIL-Demos/blob/flex_geometry/demos/4_Deep_Dives/07_flexible_geometry.ipynb
Small data set can be emailed on request.

Unit tests added for CIL api, and astra PO and FBP with circular geometry described as flex.

Related issues/links

closes #1848
closes #1849

Checklist

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have updated the relevant documentation
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License
• I confirm that the contribution does not violate any intellectual property rights of third parties