Incorrect norm in spin density for finite sampled regions

**Describe the bug**
When using the `SpinDensity` observable coupled with `cell` and `corner` keywords for sampling a finite region (not the whole simulation cell), the obtained norm from `qdens` is incorrect. For example, for an atom, if the whole simulation cell is sampled, the norm should be the total number of electrons in each spin channel (and this is currently correct). However, if one samples a [L/2, L/2, L/2] finite cell in the corner, the norm should be 1/8 = 0.125 of the original. This is currently incorrect, and the norm is still the number of electrons. So the density is being renormalized somewhere. Consider an Mg atom in a box and sampling a corner region of [L/2, L/2, L/2] (attached). With QE I can get the correct norm (0.12370), while the obtained QMC norm is 0.99716.

I tested both batched and legacy codes, and in both cases I get the incorrect norms, so the issue is likely in `qdens`. The bug can go easily unnoticed if the finite region is centered, then one expects a value close to the original norm, but this would still be incorrect.

**To Reproduce**
1. Use a cornered cell of 1/8 volume (7.55890453^3 in this case):
```

7.55890453 0.00000000 0.00000000
0.00000000 7.55890453 0.00000000
0.00000000 0.00000000 7.55890453

0 0 0

```
2. Generate the xsf file:
`qdens -f xsf -e 0 -r 1 -i vmc_J123.in.xml --noplot vmc_J123.s000.stat.h5`
3. The norm can be printed using:
`qdens-radial -p -s Mg --radii=1 -a "qmcpack" vmc_J123.s000.SpinDensity_u.xsf`

**Expected behavior**
The norm should be 1/8 of the whole cell.

**System:**
- NERSC. Mg atom in a box of L = 15.11780906 bohr.

**Additional context**
Previously, `qdens` ignored the `cell` and `corner` regions, resulting in incorrect densities. This was fixed in #4638, but evidently not everything is resolved yet. From my brief look, I am not sure where this normalization happens, but I will link a PR that will block the use of these keywords in `qdens` for now. The workaround for this bug is to sample the whole simulation space for now.

[test_corner.zip](https://github.com/user-attachments/files/17380436/test_corner.zip)