For astra and tigre ProjectionOperators, if out is passed it is filled and returned

[MargaretDuff]

Changes

For astra and tigre ProjectionOperators, if out is passed it is filled and returned. Previously it just filled. I have also added unit tests to fix this

Testing you performed

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

from cil.processors import TransmissionAbsorptionConverter, Slicer
from cil.plugins.tigre import ProjectionOperator
from cil.utilities import dataexample

# Load data
ground_truth = dataexample.SIMULATED_SPHERE_VOLUME.get()

data = dataexample.SIMULATED_CONE_BEAM_DATA.get()

# Consider just a single 2D slice 
data = data.get_slice(vertical='centre')
ground_truth = ground_truth.get_slice(vertical='centre')

absorption = TransmissionAbsorptionConverter()(data)
absorption = Slicer(roi={'angle':(0, -1, 5)})(absorption)

ig = ground_truth.geometry

A = ProjectionOperator(image_geometry=ig, 
                       acquisition_geometry=absorption.geometry)
A_scaled=3*A

Also tested on CIL-demos, week 2, SPDHG notebook.

Related issues/links

Closes #2058

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

[MargaretDuff]

@gfardell - would be grateful for your advice on if I tested everything I needed to. I possibly need to test cpu/gpu projector variations?