Replace error with warning in chain_sim()

[jamesmbaazam]

This PR will cause a minor behaviour change in chain_sim() and bump up the R version to accommodate some new base R functions in use in the package. The PR does not fix any raised issues.

Previously, whenever the user specified the serial argument in chain_sim() but also set tree = FALSE, they would get an error. In this PR, we change this behaviour so that when this argument combination occurs, we change tree to TRUE, throw a warning to inform the user about what happened and continue the simulation.

This new behaviour is intended to improve user experience, but if it causes any inconveniences or bugs, please feel free to open an issue.

[codecov-commenter]

Codecov Report

Merging #53 (5ee1d10) into main (6f3556a) will increase coverage by 3.40%.
The diff coverage is 100.00%.

❗ Current head 5ee1d10 differs from pull request most recent head 2bf83d8. Consider uploading reports for the commit 2bf83d8 to get more accurate results

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
+ Coverage   92.82%   96.23%   +3.40%     
==========================================
  Files           5        5              
  Lines         237      239       +2     
==========================================
+ Hits          220      230      +10     
+ Misses         17        9       -8     

Impacted Files	Coverage Δ
R/simulate.r	98.73% <100.00%> (+10.42%)	⬆️

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