Add defaults to all exported function arguments

[joshwlambert]

This PR addresses #120 and #145 (#146 has also been resolved in this PR, see comment below) by adding defaults to the arguments in sim_linelist(), sim_contacts() and sim_outbreaks() that were previously missing.

This now enables these functions to be run without the user specifying any arguments, e.g. linelist <- sim_linelist().

The documentation for the arguments has also be improved as suggested in #145. In combination with the argument documentation, the README, vignettes and function examples have also been added to. These now cover using a simple function call without specifying any arguments (in the README and function examples), using predefined and anonymous functions (in the simulist.Rmd vignette).

Regression unit tests have been added for the new simpler function calls for sim_*() functions.

Documentation for several internal function is also updated (mainly because of inheriting documentation from sim_*() functions).

[chartgerink]

Thanks @joshwlambert 🙏

Given that I am wrapping up before my summer holidays, I did not dive in too deep. I did request this addition so I tested the end result - and it is of great help for demo purposes to be able to run sim_linelist() and sim_contact() and get results.

Thank you again for picking up this suggestion, I appreciate it!

[joshwlambert]

This PR now also addresses #146 by not warning users if the *_risk arguments are left as their default values when onset_to_hosp or onset_to_death are set to NULL.

The onset_to_* and *_risk argument documentation has been updated to reflect this change.