Stroke capacity planning model: R DES RAP 1.0.0

Stable release, with package-level documentation, switch from R6 to function for distribution registry, validation using checkmate, and new quality assurance documentation.

New features

• Add docs/quality_assurance.md.
• Add checkmate to project dependencies.

Other changes

• Switched to package-level documentation (instead of individual @importFrom on each function).
• Changed R6 class DistributionRegistry into a function create_distribution_registry().
• Refactored validation so it uses checkmate.