Geometry utilities cast to single, casts integers to single too

This bit of code: https://github.com/CERN/TIGRE/blob/989615d8d6c33c2b88c060c14c826ff8b83cce6c/Python/tigre/utilities/geometry.py#L119-L130

## Expected Behavior
Expect anything in double precision is cast to single precision

## Actual Behavior
Casts everything to single precision including integers

## Code to reproduce the problem (If applicable)
This caused an issue when I did

```python
self.tigre_geom.cast_to_single()

img, qual= sart(
proj=self.tigre_projections,
geo=self.tigre_geom,
angles=self.tigre_angles,
init=self.tigre_initial,
niter=self.niter,
gpuids=self.gpuids,
**self.kwargs
```
because sart allocates internally based on `nVoxel` in the geometry which now isn't an integer.

## Specifications

- MATLAB/python version: python
- OS: linux
- CUDA version: 12.9

Review Python/tigre/utilities/geometry.py lines 119-130 and the cast_to_single() call path used before sart(). Reproduce the shown sart scenario and verify that double-precision values become single while integer geometry fields such as nVoxel remain integers.

Written by the indexing model from the issue text.