Fix the computation of the crop region end index

[AndersMollgaard]

Fixes #7824.

Description

The main impact of the PR is to change the computation of the end index for the extended box that RandCropBoxByPosNegLabeld uses to sample foreground centers in when self.whole_box = True. The problem and the fix is described in detail in the issue.

The PR furthermore makes a minor change to the computation of the start index of the extended box to ensure a center chosen at the margin will fully contain the original box.

I have not run the full test suite, but I have checked that I do not break the relevant unit-test: test_rand_crop_by_pos_neg_labeld.py

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [ ] Breaking change (fix or new feature that would cause existing functionality to change).
• [ ] New tests added to cover the changes.
• [ ] Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• [ ] Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• [ ] In-line docstrings updated.
• [ ] Documentation updated, tested make html command in the docs/ folder.