Fix 0.5px offset in the disc location produced by `BodyXY.centre_disc`

[ortk95]

Corrected the implementation of center_disc so that the body's disc is actually centred in the image. Previously, there was a 0.5px offset in both the x0 and y0 coordinates, so that the disc was noticeably not centred for very small images.

Old version in red, new in black:

Closes #384

Pull request checklist

• [x] Add a clear description of the change
• [x] Add any new tests needed
• [x] Run spell check on new text visible to user (documentation, GUI etc.)
• [x] Check any changes to requirements.txt are reflected in setup.py and conda-forge feedstock
• [x] Check code passes CI checks (run run_ci.sh or check GitHub Actions)

See CONTRIBUTING.md for more details.

[coveralls]

Pull Request Test Coverage Report for Build 9994685463

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.615%

---

Totals
Change from base Build 9994129195:	0.0%
Covered Lines:	3135
Relevant Lines:	3147

---

💛 - Coveralls