Scatter forward projector setter

[robbietuk]

Changes in this pull request

Introduces the ability to set and retrieve a forward projector for scatter estimation, used in mask forward projection, in the ScatterEstimation class. Also adds a keymap parser.

Related issues

Fixes #1527

Checklist before requesting a review

• I have performed a self-review of my code
• I have added docstrings/doxygen in line with the guidance in the developer guide
• [] I have implemented unit tests that cover any new or modified functionality (if applicable)
• The code builds and runs on my machine
• documentation/release_XXX.md has been updated with any functionality change (if applicable)

[robbietuk]

Ready to merge.

[KrisThielemans]

ah, sorry, can you add a line to release_6.3.htm please? (include [ci skip] in the commit, as it seems our "exclude *htm doesn't work for GitHub Actions)

[KrisThielemans]

I'll squash-merge if ok

[KrisThielemans]

looks like there's a conflict with master in the release notes...