Stroke capacity planning model: R DES RAP 0.3.0

Introduces a code of conduct and new test, and includes improvements to documentation, contributor and parameter use.

New features

• Add CODE_OF_CONDUCT.md.
• Add test for model with no arrivals.

Other changes

• Greatly improved/expanded CONTRIBUTING.md.
• Add Tom to CITATION.cff and DESCRIPTION.
• Corrections to docstrings (e.g., add missing @importFrom).
• verbose is local parameter in model rather than part of param list.
• Improvements to README.md.