finalsize 0.2.1

This patch adds:

1. Default values for the contact matrix, susceptibility matrix, and susceptibility distribution matrix to allow quick estimates of final size for a given $R_0$ in a population with homogeneous mixing and full susceptibility (#180);

2. Combines the documentation for the functions r_eff(), r0_to_lambda(), and lambda_to_r0() under the name 'r0_conversions';

3. Adds vignettes on the theoretical background and on projecting re-emergence of a disease due to demographic turnover (#193, #194 by @adamkucharski);

4. Streamlines the examples in the Readme;

5. Replaces the PNG logo in the Readme with an SVG version;

6. Updates the GitHub Actions workflow YAMLs, the .lintr config file, _pkgdown.yml, .Rbuildignore, CITATION.cff and CRAN-SUBMISSION; workflows and configuration files bring the package in line with {packagetemplate} (#181, #186, #188).

7. Standardises the DESCRIPTION file.

8. Adds package-level documentation.

9. Adds spellchecking and the WORDLIST file.

10. Adds tools to check that the package does not modify the global state, code to flag any partial matching of function arguments,.

11. Updates references cited in the vignettes.