Add default plot functions for other biokinetics outputs

[hillalex]

Adds default plot methods for objects returned from:

• simulate_individual_trajectories
• population_stationary_points

Closes #22

[codecov]

Codecov Report

Attention: Patch coverage is 97.05882% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@cf7a1e2). Learn more about missing BASE report.
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
R/plot.R	96.80%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #32   +/-   ##
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  Coverage        ?   78.79%           
=======================================
  Files           ?       11           
  Lines           ?      816           
  Branches        ?        0           
=======================================
  Hits            ?      643           
  Misses          ?      173           
  Partials        ?        0           

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