Fix bug with show2D and 3D DataContainers

[lauramurgatroyd]

Describe your changes

Fixes bug which wouldn't allow 3D DataContainers to be viewed with show2D.
Note, this was just a bug with DataContainers, not with ImageData or AcquisitionData

Describe any testing you have performed

Please add any demo scripts to CIL-Demos/misc/
Tested with a 3D datacontainer
I could not see that there are any unit tests for show2D?
Added unit tests checking get_slice with a DataContainer.

Link relevant issues

Fixes #1536

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• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• 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.
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[MargaretDuff]

Looks good.

If I understand correctly, the other way to fix the bug could have been to allow the data container get slice to take the force=True kwarg without raising an error. For the ImageData and AquisitionData case, the force kwarg is used when setting the geometry for the new slice. However, this is not needed for the DataContainer as we do not require a geometry to initiatise a VectorData or DataContainer. Therefore we don't need a force kwarg in the DataContainer case?

[lauramurgatroyd]

Yes, that’s right.

With Acquisition/ImageData it tries to create a new Acquisition/ImageGeometry and Data and if it can’t, if force is set to True then it returns a DataContainer instead.

CIL/Wrappers/Python/cil/framework/framework.py

Lines 3628 to 3638 in 5406f33

	def get_slice(self,channel=None, angle=None, vertical=None, horizontal=None, force=False):
	'''
	Returns a new dataset of a single slice of in the requested direction. \
	'''
	try:
	geometry_new = self.geometry.get_slice(channel=channel, angle=angle, vertical=vertical, horizontal=horizontal)
	except ValueError:
	if force:
	geometry_new = None
	else:
	raise ValueError ("Unable to return slice of requested AcquisitionData. Use 'force=True' to return DataContainer instead.")

I can't see there's anything we can use 'force' for in the case of a DataContainer, so it doesn't make sense for get_slice to take that parameter

[MargaretDuff]

Thanks @lauramurgatroyd - happy to approve