Slice offset for image geometry

[courtois3DM]

Description

Issue 1:
When using a data geometry that is reduced (bottom half of the detector), the corresponding image geometry is reset with its center on the center beam of the cone, which is not accurate. The center of the image geometry should be placed on the central axis of the remainder of the cone beam

data_reduced = Slicer(roi={'vertical': (0,750,1)})(data)
ag_reduced=data_reduced.geometry
ig_reduced=data_reduced.geometry.get_ImageGeometry()

Issue 2:
If selecting an image geometry of a single slice in a cone beam geometry, "get_slice" should select a given slice along the vertical axis, and locate it in the correct location within the cone beam.

data_reduced = Slicer(roi={'vertical': (0,750,1)})(data)
ag_reduced=data_reduced.geometry
ig_reduced=data_reduced.geometry.get_ImageGeometry()
#ig_reduced.get_slice('vertical'=300)
ig_reduced.voxel_num_z=1
ig_reduced.center_z=-450*ig.voxel_size_z #(for slice 300)
show_geometry(ag_reduced,ig_reduced,elevation=0,azimuthal=0)

Environment

Python 3.12.12 | packaged by conda-forge | (main, Oct 13 2025, 14:34:15) [GCC 14.3.0] on linux

[lauramurgatroyd]

#2235 should resolve issue 2 but I'll check next week!