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

• [x] I have performed a self-review of my code
• [x] 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)
• [x] The code builds and runs on my machine
• [] documentation/release_XXX.md has been updated with any functionality change (if applicable)