Simple M/M/s queuing model: R DES RAP v0.5.0

This update adds new performance measures and tests, improves the GitHub actions, implements some fixes and documentation improvements, among other changes.

New features

• Add choice of operating system when manually trigger R-CMD-check GitHub action.
• Add check in model validation for non-null file path when log_to_file = TRUE.
• Calculate test coverage.
• Environment - add covr and queueing and DT and switched to "all" snapshot type.
• Add seed_offset and ran sensitivity analysis on seed in choosing_replications.Rmd.
• Add test comparing against analytical queueing theory model from queueing package.
• Add mean time in system to performance measures.
• Add CODE_OF_CONDUCT.md.

Bug fixes

• Skip parallel processing test on Mac GitHub action.
• Fix deviation calculation so it doesn't get stuck for NA/0.
• Corrected choice of warm-up to run model for longer.
• Fixed/expanded back tests.

Other changes

• Replaced rap_template_r_des with rdesrap_mms.
• Changed all magrittr's %>% to be base R's >| (as per tidyverse style guide).
• Improved README.md and CONTRIBUTING.md.
• Simplify package loading.
• Reorganised tests.
• Update parameters to match python version of model.
• Adjusted analysis.Rmd bin widths to be more consistent with python version of analysis.
• Other minor corrections and amendments (e.g., linting, docstrings, tests).