Fix and upgrade plot_orbit

[TomWagg]

• New feature: plot_orbit now shows the location of mergers (either stellar or GW), this can be turned off with the show_merger argument
• Bug fix: plot_orbit hides the SN/merger locations if they occur after t_max (previously they were shown even though the orbit might not reach them)
• Bug fix (#154): Ensure secondary SNe are still shown for bound binaries in plot_orbit

This will fix #154

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b3a48d5) to head (f749539).
Report is 8 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #155      +/-   ##
===========================================
+ Coverage   99.13%   100.00%   +0.86%     
===========================================
  Files          15        15              
  Lines        1726      1715      -11     
===========================================
+ Hits         1711      1715       +4     
+ Misses         15         0      -15     

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