179: make D real

[sbfnk]

Description

This PR closes #179 by defining D as real allowing it to be Inf.

[Describe the changes that you made in this pull request.]

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• My code follows the established coding standards.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[seabbs]

Good catch I think a future feature/issue should be to make d, pwindow, and d_upper real as well and check this is supported as this could in theory be something someone might want to do

Can you please add a news item and if not done in the other PR add yourself as a contributor

I think ideally this would have a test to avoid it happening in the future. If you feel like it could you add?

[seabbs]

Nice LGTM. I used fewer cores and chains in the tests and made the news update a bit more specific.